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.
When they create a black hole and destroy the earth, they can say "but it was such a cool experiment..."
Isn't this when the military steps in and offers additional funding?
Adding features does not necessarily increase functionality -- it just makes the manuals thicker.