How did pictures of scantily clad, apparently underaged girls end up used without permission in an ad campaign on Facebook? Blame affiliate advertising, and the limits of the social network's ad screening.

Facebook approves every advertisement on its site. But it didn't notice a watermark on one racy ad's picture, identifying it, Forbes says, as one from Jailbaitgallery.com, where the users had guessed the age of the two girls in the photo to be approximately 16 years. Jailbait Gallery apparently specializes in those sorts of votes. Lovely.

The ad promoted alumni networking site MyLife.com, and, according to Forbes' Taylor Buley, "showed two apparently underage blondes in low-cut shirts." MyLife ads on Facebook's current ad board, like the one above, still feature racy female pictures, but presumably of adults: The MyLife executive in charge of affiliate marketers, like the one who took out the offending ad, told Forbes, "a very small fraction of 1% of [our marketing] traffic... would be promoting those kinds of images."

He added, unimpressively, "We've been trying to get our arms around the whole policing aspect." Uh, OK. But surely Facebook is swearing up and down this won't happen again? "A spokesman says copyright owners can fill out an online form and Facebook will take action within 24 hours."

Got that, underaged girls? The best way to keep your pictures from being abused on Facebook is to be sure you're constantly reloading Facebook, according to Facebook.

(Image: A different Facebook ad for MyLife.com, without the "Jailbaitgallery.com" logo and presumably featuring an adult model. Via Facebook.)