A group of security researchers at the University of Cambridge have discovered that some social networking sites keep all the photos you upload and then delete.
16 websites, including Facebook, MySpace, Bebo, Orkut, Blogger and Flickr had pictures uploaded to them. The researchers noted the URLs of the pictures, and then 'deleted' them from the site. They found that after 30 days, seven of the 16 sites were still serving the pictures.
The offenders were Bebo, Facebook, hi5, LiveJournal, MySpace, SkyRock and Xanga. Other sites took some time to take them down. Only on Flickr, Orkut, Photobucket and Windows Live Spaces did the changes have immediate effect.
The researchers say that for many sites, "photos are deleted (...) too infrequently to justify the overhead and complexity of removing them from the content delivery network". They quote Larry Lessig in saying "privacy is still considered a matter of law and not of code".
It's caused the sites in question some consternation. Xanga and Livejournal have publicly responded saying that their systems shouldn't be behaving that way, and asking the researchers to contact them with more details on how to reproduce the issues.
But for the moment, our advice would be to not upload anything that you don't think you'll want viewable in the future. It may stay online longer than you think.