Ten privacy campaign groups have written to the FTC calling for an investigation into Facebook's use of so-called supercookies.
They say they're concerned about the company tracking users even after they've logged out of the service - and claim it's failed to carry out promises to ensure user privacy.
"Facebook‟s tracking of post-log-out Internet activity violates both the reasonable expectations of consumers and the company‟s own privacy statements," they say in their letter.
"Although Facebook has partially fixed the problem caused by its tracking cookies, the company still places persistent identifiers on users' browsers that collect post-log-out data and could be used to identify users."
Quite apart from the issue of supercookies, another problem, say the groups, is Facebook's new 'frictionless sharing' feature. This allos apps to post user activity in real time, without asking user permission for each update.
"“Frictionless sharing” plays a leading role in the changes Facebook announced at the recent f8 development conference, and works through the interaction of Facebook‟s Ticker, Timeline, and Open Graph," reads the letter.
"These changes in business practices give the company far greater ability to disclose the personal information of its users to its business partners than in the past. Options for users to preserve the privacy standards they have established have become confusing, impractical, and unfair."
The letter's signed by the the Electronic Privacy Information Center, The American Civil Liberties Union, the American Library Association, the Bill of Rights Defense Committee, The Center for Digital Democracy, the Center for Media and Democracy, Consumer Action, Consumer Watchdog, Privacyactivism, and Privacy Times.
Meanwhile, the data protection minister in Ireland - home of Facebook's European headquarters - is to carry out a privacy audit of the company's activities outside North America. The investigation follows a complaint from an Austrian group called Europe versus Facebook.