More than just backups. When you "delete" something your just setting a Is_Deleted flag on their database. As far as facebook is concerned, your information is just as easily available as if you were an active member.
What if you simply change it, say, to something bogus?
They might have some kind of version control system, but a version control system is really a kind of backup with a particular purpose.
This is exactly what you should do. Except that you should phase it in slowly, and the new data should be reasonable (read: believable). The only way you can fight with them having your data and refusing to delete it is to make their data useless by polluting it with noise.
So the other day I messaged another admin from the console using the regular old 'write' command (as I've been doing for over 10 years). To my surprise he didn't know how to respond back to me (he had to call me on the phone) and had never even known you could do that. That got me thinking that there's probably lots of things like that, and likely things I've never heard of. What sorts of things do you take for granted as a natural part of Unix that other people are surprised at?
Saliva causes cancer, but only if swallowed in small amounts over a long period of time. -- George Carlin