A would-be class of potentially millions of Facebook users over the age of 40 who were excluded from seeing ads for certain U.S. job opportunities is overbroad and cannot be sustained, several major companies said in new court papers filed Monday in California.

The defendants, including Amazon.com, T-Mobile US and Cox Media Group, are fighting the plaintiffs' effort to certify a class in a novel age-discrimination lawsuit in San Jose federal court. Facebook Inc. was not sued.

The challengers—the Communications Workers of America and three named plaintiffs—contend the companies discriminated against potential older job applicants by targeting ads exclusively to a younger demographic. The lawsuit includes claims under the federal Age Discrimination in Employment Act as well as California state laws.

Lawyers for the three companies—Gibson, Dunn & Crutcher for Amazon; Winston & Strawn for Cox; and Davis Wright Tremaine for T-Mobile—contend the proposed class, which could include “nearly everyone over the age of 40 with a Facebook account,” is overbroad, vague and subjective. The attorneys also argue the “myriad individualized inquiries would overrun common questions, most notably because plaintiffs seek to apply the laws of multiple jurisdictions to millions of individuals, each of whom has her own qualifications, employment history, and application history.”

Jason Schwartz, a Gibson Dunn partner in Washington representing Amazon, declined to comment. Amazon also filed a motion to dismiss the lawsuit Monday.

Facebook's advertising platform has come under increased scrutiny from U.S. lawmakers and the AARP. In addition to the class action against Amazon and the other companies, the law firm Outten & Golden has filed dozens of age-bias allegations with the U.S. Equal Employment Opportunity Commission against Facebook and the companies that use the platform.

Facebook's platforms have long drawn criticism. The company last week agreed to make changes to an ad tool to resolve a discrimination investigation led by the Washington attorney general. Facebook removed the ability of advertisers to exclude certain protected groups, including ethnic and religious minorities, from seeing ads.

The Amazon lawsuit in the U.S. District Court for the District of Northern California asks the court to declare that the companies' advertising and recruitment practices violate federal law. The complaint seeks an injunction that would stop companies from using the Facebook platform to discriminate against older job applicants.

Outten & Golden attorney Peter Romer-Friedman said the case is “emblematic of what the future looks like in employment law.”

“Employers use online and big data tools to do a ton of their decision-making and recruiting,” Romer-Friedman said Monday. “We are planning to stay on our toes when employers use these tools to discriminate.”

He said the case would present “opt-in” opportunities if the class is granted. That means the class could include any user, over 40, who would have been interested in a job opportunity advertised by the defendant companies. That could mean thousands to tens of millions of plaintiffs, he said.

Romer-Friedman said the class is certifiable because each defendant made the same argument that they were interested in reaching younger workers and “each and every company expressly excluded the workers from seeing the ads.”

“If this case can't be certified, it would be an unfortunate commentary on the federal courts power to stop discriminatory practices,” Romer-Friedman said. “This is an important case to ensure that digital recruitment and the digital economy provides equal opportunity for all who work.”

A hearing is scheduled for Jan. 24 in San Jose before U.S. District Judge Beth Labson Freeman.

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