The research done by the Cambridge University's guys have found that everything the user deletes is actually not deleted(however it stops showing on your gallery). Not only Facebook but nearly half the social networking websites don't immediately delete any files that are requested to be deleted by the user. Any way Flicker is said to do a better job on deleting the pictures faster.
So why is the deleted photos stick on so long? The problem is the way data is stored on large websites. ie, Our PC keeps only keeps one copy of a file, but the large scale services rely on what are called content delivery networks to manage data and distribution. In this system the content is copied to multiple intermediate devices, usually to speed up the access to files when millions of peoples are trying to access simultaneously. But because changes aren't reflected across the system immediately, ghost copies of files tend to linger for days or weeks.
In the case of Facebook, the company says data may hang around until the URL in question is reused, which is usually "after a short period of time." Though obviously that time can vary considerably.
Of course, once a photo escapes from the walled garden of a social network like Facebook, the chances of deleting it permanently fall even further. Google's caching system is remarkably efficient at archiving copies of web content, long after it's removed from the web. Anyone who's ever used Google Image Search can likely tell you a story about clicking on a thumbnail image, only to find that the image has been deleted from the website in question -- yet the thumbnail remains on Google for months. And then there are services like the Wayback Machine, which copy entire websites for posterity, archiving data and pictures forever.How do you feel that?? Do think twice when uploading your wacky pictures(only if).