Amazon, T-Mobile, and Cox Communications are among hundreds of companies facing a new lawsuit for allegedly excluding older workers from receiving job ads served on Facebook.

"For employers and employment agencies that want to exclude older workers, Facebook’s ad platform is a blessing," states the complaint, filed this week by the Communications Workers of America union and three job seekers over the age of 40. "Rather than promoting non-discrimination in employment, Facebook’s services, ad platform, and tools, used by employers and employment agencies, have perpetuated age discrimination in employment nationwide, and have greatly diminished the employment opportunities of older Americans."

The lawsuit, filed in U.S. District Court for the Northern District of California, was brought the same day ProPublica reported that Facebook allows advertisers to target job ads based on age. The Communications Workers and other plaintiffs allege that Amazon, Cox and the other advertisers are violating anti-discrimination laws in the District of Columbia and a host of states.

The union alleges in a class-action that it has found more than 100 employers and employment agencies that exclude older workers from job ads on Facebook. The complaint specifically references ads placed by 13 companies, including Facebook itself, which allegedly targeted job ads to people younger than 61.

Amazon allegedly restricted ads to Facebook users between 18 and 55, while T-Mobile allegedly advertised jobs to people between 18 and 54, and Cox arranged to prevent its job ads from being shown to users older than 55. Amazon reportedly said Thursday it has "corrected" ads that discriminate based on age.

"Twenty years ago, an employer would have had to go to great lengths -- at a great cost -- to determine the age of all potential applicants so that the employer could exclude older workers from its advertising and recruitment," the complaint states. "But today Facebook does exactly that before a single ad has been purchased or sent."

The union's lawsuit seeks economic damages and an injunction prohibiting companies from excluding older workers from receiving job ads.

Some employers who use Facebook's age-targeting options for job ads may be inviting trouble, according to Santa Clara University law professor Eric Goldman. He says employers may be justified in using age targeting if there are "exceptional circumstances," but that many age-targeted job ads will be problematic.

"There are circumstances where it would make sense for an employer to use the age targeting field. For example,there are jobs that might have a minimum or maximum age, so it wouldn't make any sense to target people outside of that bracket," he says. Goldman adds that employers also can legitimately use age targeting if they're targeting all age groups, but want to tailor the messages.

"It seems like those are going to be the exceptional circumstances," he says. "Most of the ads won't meet either of those two parameters, and the age-targeting field will only be litigation bait."