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.
and then probably facebook will say: "ehi B, where did you buy those FBs?? You are allowed to buy only from us. Your account is closed."
They can do that.