A proposed Facebook policy that would allow your picture to be used in a product or store's ad on the site has privacy groups united in opposition.

Executives from six public interest groups on Wednesday signed a letter to the Federal Trade Commission, asking that the agency block the practice.

"Facebook (FB) users who reasonably believed that their images and content would not be used for commercial purposes without their consent will now find their pictures showing up on the pages of their friends endorsing the products of Facebook's advertisers," said the letter. "Remarkably, their images could even be used by Facebook to endorse products that the user does not like or even use."

Facebook reached a $20 million settlement last week in a class action suit brought by those who argued it did not have enough privacy protections.

Facebook said the proposed policy was in response to that settlement. It said it would listen to comments over a 7-day period that ends Thursday, and that it would consider feedback before adopting the changes.

In response to questions about the objections, Facebook said Thursday that it has not changed its ad practices or policies, and that it is only making things clearer for people who use the service. It said it wants to make clear that you are granting Facebook permission for this use when you use its services.

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But Jeffrey Chester, executive director for the Center for Digital Democracy and one of the signers of the letter, said the new policy opens the door for even greater user of user's data.

"It requires 'Alice in Wonderland' logic to see this as anything but a major setback for the privacy rights of Facebook users," said the group's letter. The group is particularly upset because it said it makes it easier to use the images of minors who use the site.

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Chester said Facebook unveiled the new rules on the eve of the Labor Day holiday in hopes it could be put in place before the FTC and public had a chance to object. But the comments from users at the bottom of the page announcing the changes were overwhelmingly negative.

"Send me ads, fine. Use my photos or posts in ads, I'm out of here," wrote Margo Kelly, one of the Facebook users posting a comment.

Among the other groups whose executives signed the letter are the Electronic Privacy Information Center, the U.S. Public Interest Research Group and the Privacy Rights Clearinghouse.