Updated 8:27 a.m.

Facebook's advertising platforms unlawfully discriminate against women by enabling employers to target only men for job opportunities, civil rights advocates said Tuesday in claims filed with the U.S. Equal Employment Opportunity Commission.

The company's advertising practices have come under greater scrutiny for alleged violations of federal employment and housing laws. The new claims, brought against Facebook and 10 companies, focus on alleged gender discrimination. Excluding protected classes—based on age, race, sex, national origin and religion—raises potential civil rights liability.

The allegations, on behalf of three female workers, include claims that millions of women were denied information about job opportunities because of their gender. The ACLU, Outten & Golden and the Communications Workers of America filed charges with the EEOC, a necessary first step before any lawsuit.

“Sex segregated job advertising has historically been used to shut women out of well-paying jobs and economic opportunities,” Galen Sherwin, a senior staff attorney at the ACLU's Women's Rights Project, said in a statement. “We can't let gender-based ad targeting online give new life to a form of discrimination that should have been eradicated long ago.”

A Facebook spokesperson said Tuesday in a statement: “There is no place for discrimination on Facebook; it's strictly prohibited in our policies, and over the past year, we've strengthened our systems to further protect against misuse. We are reviewing the complaint and look forward to defending our practices.”

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Outten & Golden attorneys and the Communication Workers of America have sued companies in California court over Facebook advertising that allegedly discriminates against older workers. Facebook is not a named defendant in that case.

Lawyers for the companies—including Amazon.com Inc., represented by Gibson, Dunn & Crutcher—have filed court papers urging a judge to dismiss the claims.

Facebook last month announced it would remove 5,000 advertising targeting options “to help prevent misuse.”

“While these options have been used in legitimate ways to reach people interested in a certain product or service, we think minimizing the risk of abuse is more important. This includes limiting the ability for advertisers to exclude audiences that relate to attributes such as ethnicity or religion,” the company said in a statement. The changes did not limit advertising for age or sex.

The spotlight on Facebook's advertising platform reveals blurry lines in the modern age. Hundreds of millions of people use Facebook, and companies can target product messages, employment and other opportunities to specific groups of users. Facebook and other tech companies contend in many instances they are immune—through the federal Communications Decency Act—for the content posted by third parties.

The U.S. Department of Housing and Urban Development recently brought an administrative housing discrimination complaint against Facebook. The company, according to the complaint, unlawfully discriminates “by enabling advertisers to restrict which Facebook users receive housing-related ads based on race, color, religion, sex, familial status, national origin and disability.”

HUD's complaint also noted that Facebook's ad targeting tools unlawfully allow advertisers “to discriminate based on sex by showing ads only to men or only to women.”

The civil rights advocates and lawyers for the workers are targeting Facebook as an employment agency.

“The internet did not erase our civil rights laws. It violates the law if an employer uses Facebook to deny job ads to women,” said Peter Romer-Friedman, an Outten & Golden counsel in Washington. “The last time I checked, you don't have to be a man to be a truck driver or a police officer. But Facebook and employers are acting like it's the 1950s, before federal employment law banned sex discrimination.”

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