Just because you've deleted a picture form your profile on facebook (for example) doesn't mean that the file is actually removed from their servers. All you've done is remove the link to the picture from the profile. Chances are that each individual picture is replicated over a nuber of storage devices on the facebook servers to proivde for failure, huigher speed access etc etc. And data will be backed up to tape, disc or offline discs to enable a restoration of service.
So facebook will have multiple copies of that one file that you've uploaded to it's servers. How many people have downloaded the picture that you've uploaded too? There could be copies anywhere ... I've seen photos I've taken (some of which were only ever shared briefly on this or other sites) appear on other peoples profiles for example.
The only true way to delete anything is to have it physically erased from the hard disc that it's stored on, and then to have that portion of the disc overwritten with random data a number of times (7 times is generally accepted as enough to ensure that the data is not accessible to anyone. The backup tapes, discs or offline storage arrays would need cleansing too, which is much more complex.
The same holds true with your hard drive on your PC at home or your memory stick or your memory card in your smart phone. Just because you've "deleted it" and then emptied your wastebasket, doesn't mean it's really gone. Give me 5 minutes and I could have it back again
