When I deleted my account in 2010, there was a one month cooling off period, during which any sign-in would cancel the deletion request.
Perhaps that's sensible to catch malicious/accidental deletions, but I imagine it also makes it hard for Facebook addicts to leave.
Especially when during those few weeks you get constantly bothered by emails like 'your friends are missing you', 'look what they posted this week', etc. and when you click on anything - bam, you're logged back.
You can do this by changing your password right before you delete your FB account, as that is supposed to revoke OAuth tokens.
So it's essentially a backup in case my address book fails. I'll pop in there maybe a dozen times a year. That's not exactly using their service.