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.
"If they are transferable between individual users" is what I said. That would have to be a facility provided by facebook. If they don't want you to do that then that hypothetical will be false, and the rest of what I said would obviously not be correct.
Transfers between users implies trade is allowed; bitcoins are tradeable. If one, then the other.