Hi anonymous, I work at facebook. If you manage to hack us and grab our data, perhaps you'd like to apply for a job? It's not trivial getting the data even with full access, so you could really help our team! Plus the free lunches are pretty good.
If I were them and wanted the data I'd hack a big Facebook application' account and since most of them are very intrusive with their permissions I'd get a lot of data that way.
I bet from the backend it'd be very hard to do it, and the data might be encrypted using a key that only a Facebook user has (the user might have multiple keys and share them with friends... etc etc), anyways, I'm thinking too much about this, it's too late already :).