The Federal Trade Commission says that Facebook is misinterpreting a key children's privacy law, in a move that could weaken the social network's argument in a California district court suit over teen privacy on the Web site.

The FTC filed the brief Thursday night, weighing in on a key point for the case, Batman v. Facebook (also known as Fraley v. Facebook). In it, the FTC says Facebook is wrong to say that, because the Children's Online Privacy and Protection Act (COPPA) only protects the privacy of children under 12, that the law could be interpreted to keep states from enforcing their own laws on teen privacy.

This was a main part of Facebook’s argument against those who object to a 2012 settlement over “Sponsored Stories,” which featured users’ “likes” and check-ins in advertisement. The users say the $10 million settlement does not require teens to get explicit permission from their parents before agreeing to show up in advertisements, in violation of state teen privacy laws in seven states, including Virginia.

In a court filing in June, Facebook argued in part that because COPPA protections stop at age 12, that could be read as a clear message that "teenagers' Internet activities should not be subject to parental consent requirements, even under the auspices of state law."

But that idea, the FTC said in its own filing, is "wrong and should be rejected."

"Nothing in the language, structure of legislative history," the brief said, can be interpreted to "displace state protections of teenagers' online privacy in their entirety."

The agency said it had no position to take on the merits of the larger case.

Privacy advocates from the Center for Digital Democracy and Public Citizen had publicly called on the agency to challenge Facebook's interpretation after the settlement, and celebrated the agency's brief.

Scott Michelman, staff attorney for the consumer advocacy group Public Citizen, said that the FTC's action -- even without offering its support to either side of the case -- undercuts a key part of the social network's argument.

Jeffrey Chester, executive director of the Center for Digital Democracy, went even further, saying, "This struck a blow against Facebook's plans."

The brief does not address other issues in the case, such as whether anyone was harmed by having their picture and name publicly linked in advertisements for products they digitally endorsed on their profile. Facebook has since discontinued the "Sponsored Stories" product, but still can display users' names and photos alongside posts showing what they've liked -- publicly, if users allow that through their privacy settings.

In a statement, a Facebook spokeswoman said, "The district court correctly found that the settlement was a fair and reasonable resolution of the claims in this case after fully considering these issues."

The state of California on Friday also filed an amicus brief affirming its right to enforce its state privacy law.