Try this: Take a photo and upload it to Facebook, then after a day or so, note what the URL to the picture is (the actual photo, not the page on which the photo resides), and then delete it. Come back a month later and see if the link works. Chances are: It will.

Facebook isn't alone here. Researchers at Cambridge University (so you know this is legit, people!) have found that nearly half of the social networking sites don't immediately delete pictures when a user requests they be removed. In general, photo-centric websites like Flickr were found to be better at quickly removing deleted photos upon request.

Why do "deleted" photos stick around so long? The problem relates to the way data is stored on large websites: While your personal computer only keeps one copy of a file, large-scale services like Facebook rely on what are called content delivery networks to manage data and distribution. It's a complex system wherein data is copied to multiple intermediate devices, usually to speed up access to files when millions of people are trying to access the service simultaneously. (Yahoo! Tech is served by dozens of servers, for example.) But because changes aren't reflected across the CDN immediately, ghost copies of files tend to linger for days or weeks.

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