A few weeks ago, Verizon placed an ad on Facebook to recruit applicants for a unit focused on financial planning and analysis. The ad showed a smiling, millennial-aged woman seated at a computer and promised that new hires could look forward to a rewarding career in which they would be “more than just a number.”

Some relevant numbers were not immediately evident. The promotion was set to run on the Facebook feeds of users 25 to 36 years old who lived in the nation’s capital, or had recently visited there, and had demonstrated an interest in finance. For a vast majority of the hundreds of millions of people who check Facebook every day, the ad did not exist.

Verizon is among dozens of the nation’s leading employers — including Amazon, Goldman Sachs, Target and Facebook itself — that placed recruitment ads limited to particular age groups, an investigation by ProPublica and The New York Times has found.

The ability of advertisers to deliver their message to the precise audience most likely to respond is the cornerstone of Facebook’s business model. But using the system to expose job opportunities only to certain age groups has raised concerns about fairness to older workers.

Several experts questioned whether the practice is in keeping with the federal Age Discrimination in Employment Act of 1967, which prohibits bias against people 40 or older in hiring or employment. Many jurisdictions make it a crime to “aid” or “abet” age discrimination, a provision that could apply to companies like Facebook that distribute job ads.

“It’s blatantly unlawful,” said Debra Katz, a Washington employment lawyer who represents victims of discrimination.

Facebook defended the practice. “Used responsibly, age-based targeting for employment purposes is an accepted industry practice and for good reason: it helps employers recruit and people of all ages find work,” said Rob Goldman, a Facebook vice president.

The revelations come at a time when the unregulated power of the tech companies is under increased scrutiny, and Congress is weighing whether to limit the immunity that it granted to tech companies in 1996 for third-party content on their platforms.

Facebook has argued in court filings that the law, the Communications Decency Act, makes it immune from liability for discriminatory ads.

Although Facebook is a relatively new entrant into the recruiting arena, it is rapidly gaining popularity with employers. Earlier this year, the social network launched a section of its site devoted to job ads. Facebook allows advertisers to select their audience, and then Facebook finds the chosen users with the extensive data it collects about its members.

The use of age targets emerged in a review of data originally compiled by ProPublica readers for a project about political ad placement on Facebook. Many of the ads include a disclosure by Facebook about why the user is seeing the ad, which can be anything from their age to their affinity for folk music.

The precision of Facebook’s ad delivery has helped it dominate an industry once in the hands of print and broadcast outlets. The system, called microtargeting, allows advertisers to reach essentially whomever they prefer, including the people their analysis suggests are the most plausible hires or consumers, lowering the costs and vastly increasing efficiency.

Targeted Facebook ads were an important tool in Russia’s efforts to influence the 2016 election. The social media giant has acknowledged that 126 million people saw Russia-linked content, some of which was aimed at particular demographic groups and regions. Facebook has also come under criticism for the disclosure that it accepted ads aimed at “Jew-haters” as well as housing ads that discriminated by race, gender, disability and other factors.

Other tech companies also offer employers opportunities to discriminate by age. ProPublica bought job ads on Google and LinkedIn that excluded audiences older than 40 — and the ads were instantly approved. Google said it does not prevent advertisers from displaying ads based on the user’s age. After being contacted by ProPublica, LinkedIn changed its system to prevent such targeting in employment ads.

The practice has begun to attract legal challenges. On Wednesday, a class-action complaint alleging age discrimination was filed in federal court in San Francisco on behalf of the Communications Workers of America and its members — as well as all Facebook users 40 or older who may have been denied the chance to learn about job openings. The plaintiffs’ lawyers said the complaint was based on ads for dozens of companies that they had discovered on Facebook.

The database of Facebook ads collected by ProPublica shows how often and precisely employers recruit by age. In a search for “part-time package handlers,” United Parcel Service ran an ad aimed at people 18 to 24. State Farm pitched its hiring promotion to those 19 to 35.

Some companies, including Target, State Farm and UPS, defended their targeting as a part of a broader recruitment strategy that reached candidates of all ages. The group of companies making this case included Facebook itself, which ran career ads on its own platform, many aimed at people 25 to 60. “We completely reject the allegation that these advertisements are discriminatory,” said Goldman of Facebook.

After being contacted by ProPublica and the Times, other employers, including Amazon, Northwestern Mutual and the New York City Department of Education, said they had changed or were changing their recruiting strategies.

“We recently audited our recruiting ads on Facebook and discovered some had targeting that was inconsistent with our approach of searching for any candidate over the age of 18,” said Nina Lindsey, a spokeswoman for Amazon, which targeted some ads for workers at its distribution centers between the ages of 18 and 50. “We have corrected those ads.”

Verizon did not respond to requests for comment.

Several companies argued that targeted recruiting on Facebook was comparable to advertising opportunities in publications like the AARP magazine or Teen Vogue, which are aimed at particular age groups. But this obscures an important distinction. Anyone can buy Teen Vogue and see an ad. Online, however, people outside the targeted age groups can be excluded in ways they will never learn about.

“What happens with Facebook is you don’t know what you don’t know,” said David Lopez, a former general counsel for the Equal Employment Opportunity Commission who is one of the lawyers at the firm Outten & Golden bringing the age-discrimination case on behalf of the communication workers union.

‘They Know I’m Dead’

Age discrimination on digital platforms is something that many workers suspect is happening to them, but that is often difficult to prove.

Mark Edelstein, a fitfully employed social-media marketing strategist who is 58 and legally blind, doesn’t pretend to know what he doesn’t know, but he has his suspicions.

Edelstein, who lives in St. Louis, says he never had serious trouble finding a job until he turned 50. “Once you reach your 50s, you may as well be dead,” he said. “I’ve gone into interviews, with my head of gray hair and my receding hairline, and they know I’m dead.”

Edelstein spends most of his days scouring sites like LinkedIn and Indeed and pitching hiring managers with personalized appeals. When he scrolled through his Facebook ads on a Wednesday in December, he saw a variety of ads reflecting his interest in social media marketing: ads for the marketing software HubSpot (“15 free infographic templates!”) and TripIt, which he used to book a trip to visit his mother in Florida.

What he didn’t see was a single ad for a job in his profession, including one identified by ProPublica that was being shown to younger users: a posting for a social media director job at HubSpot. The company asked that the ad be shown to people aged 27 to 40 who live or were recently living in the United States.

“Hypothetically, had I seen a job for a social media director at HubSpot, even if it involved relocation, I ABSOLUTELY would have applied for it,” Edelstein said by email when told about the ad.

A HubSpot spokeswoman, Ellie Botelho, said that the job was posted on many sites, including LinkedIn, The Ladders and Built in Boston, and was open to anyone meeting the qualifications regardless of age or any other demographic characteristic.

She added that “the use of the targeted age-range selection on the Facebook ad was frankly a mistake on our part given our lack of experience using that platform for job postings and not a feature we will use again.”

For his part, Edelstein says he understands why marketers wouldn’t want to target ads at him: “It doesn’t surprise me a bit. Why would they want a 58-year-old white guy who’s disabled?”

Looking for ’Younger Blood’

Although LinkedIn is the leading online recruitment platform, according to an annual survey by SourceCon, an industry website. Facebook is rapidly increasing in popularity for employers.

One reason is that Facebook’s sheer size — two billion monthly active users, versus LinkedIn’s 530 million total members — gives recruiters access to types of workers they can’t find elsewhere.

Consider nurses, whom hospitals are desperate to hire. “They’re less likely to use LinkedIn,” said Josh Rock, a recruiter at a large hospital system in Minnesota who has expertise in digital media. “Nurses are predominantly female, there’s a larger volume of Facebook users. That’s what they use.”

There are also millions of hourly workers who have never visited LinkedIn, and may not even have a résumé, but who check Facebook obsessively.

Deb Andrychuk, chief executive of the Arland Group, which helps employers place recruitment ads, said clients sometimes asked her firm to target ads by age, saying they needed “to start bringing younger blood” into their organizations. “It’s not necessarily that we wouldn’t take someone older,” these clients say, according to Andrychuk, “but if you could bring in a younger set of applicants, it would definitely work out better.”

Andrychuk said that “we coach clients to be open and not discriminate” and that after being contacted by The Times, her team updated all their ads to ensure they didn’t exclude any age groups.

But some companies contend that there are permissible reasons to filter audiences by age, as with an ad for entry-level analyst positions at Goldman Sachs that was distributed to people 18 to 64. A Goldman Sachs spokesman, Andrew Williams, said showing it to people above that age range would have wasted money: roughly 25 percent of those who typically click on the firm’s untargeted ads are 65 or older, but people that age almost never apply for the analyst job.

“We welcome and actively recruit applicants of all ages,” Williams said. “For some of our social-media ads, we look to get the content to the people most likely to be interested, but do not exclude anyone from our recruiting activity.”

Pauline Kim, a professor of employment law at Washington University in St. Louis, said the Age Discrimination in Employment Act, unlike the federal anti-discrimination statute that covers race and gender, allows an employer to take into account “reasonable factors” that may be highly correlated with the protected characteristic, such as cost, as long as they don’t rely on the characteristic explicitly.

The Question of Liability

In various ways, Facebook and LinkedIn have acknowledged at least a modest obligation to police their ad platforms against abuse.

Earlier this year, Facebook said it would require advertisers to “self-certify” that their housing, employment and credit ads were compliant with anti-discrimination laws, but that it would not block marketers from purchasing age-restricted ads.

Still, Facebook didn’t promise to monitor those certifications for accuracy. And Facebook said the self-certification system, announced in February, was still being rolled out to all advertisers.

LinkedIn, in response to inquiries by ProPublica, added a self-certification step that prevents employers from using age ranges when placing an employment ad, unless they affirm the ad is not discriminatory.

With these efforts evolving, legal experts say it is unclear how much liability the tech platforms could have. Some civil rights laws, like the Fair Housing Act, explicitly require publishers to assume liability for discriminatory ads.

But the Age Discrimination in Employment Act assigns liability only to employers or employment agencies, like recruiters and advertising firms.

The lawsuit filed against Facebook on behalf of the communications workers argues that the company essentially plays the role of an employment agency — collecting and providing data that helps employers locate candidates, effectively coordinating with the employer to develop the advertising strategies, informing employers about the performance of the ads, and so forth.

Regardless of whether courts accept that argument, the tech companies could also face liability under certain state or local anti-discrimination statutes. For example, California’s Fair Employment and Housing Act makes it unlawful to “aid, abet, incite, compel or coerce the doing” of discriminatory acts proscribed by the statute.

“They may have an obligation there not to aid and abet an ad that enables discrimination,” said Cliff Palefsky, an employment lawyer based in San Francisco.

The question may hinge on Section 230 of the federal Communications Decency Act, which protects internet companies from liability for third-party content.

Tech companies have successfully invoked this law to avoid liability for offensive or criminal content — including sex trafficking, revenge porn and calls for violence against Jews. Facebook is currently arguing in federal court that Section 230 immunizes it against liability for ad placement that blocks members of certain racial and ethnic groups from seeing the ads.

“Advertisers, not Facebook, are responsible for both the content of their ads and what targeting criteria to use, if any,” Facebook argued in its motion to dismiss allegations that its ads violated a host of civil rights laws. The case does not allege age discrimination.

Eric Goldman, professor and co-director of the High Tech Law Institute at the Santa Clara University School of Law, who has written extensively about Section 230, says it is hard to predict how courts would treat Facebook’s age-targeting of employment ads.

Goldman said the law covered the content of ads, and that courts have made clear that Facebook would not be liable for an advertisement in which an employer wrote, say, “no one over 55 need apply.” But it is not clear how the courts would treat Facebook’s offering of age-targeted customization.

According to a federal appellate court decision in a fair-housing case, a platform can be considered to have helped “develop unlawful content” that users play a role in generating, which would negate the immunity.

“Depending on how the targeting is happening, you can make potentially different sorts of arguments about whether or not Google or Facebook or LinkedIn is contributing to the development” of the ad, said Deirdre K. Mulligan, a faculty director of the Berkeley Center for Law and Technology.