Perhaps I don't understand facebook credits enough (I know nothing about them, so that's pretty likely); but if they are transferable between individual users, then they are exchangeable for bitcoins.

A: Hello, I have 10 FB credits.
B: Hello, I have 10 BTC; would you like to trade?
A:
B:

Trust is a problem; chargebacks are a problem; but they're for someone else to solve (escrow?).  It's certainly doable though, and doesn't require permission from facebook.  Since this exchange could be organised outside facebook, they needn't even know.