They do, but Facebook is already testing out their payment system. The thing is Facebook customers trust Facebook. Having a stored credit system which could then be used for Facebook gifts and purchases in apps would cause more than a 10% uptick in sales for app devs like Zynga, so Facebook could charge a relatively high processing fee like that and still be worth using. They don't even need to mandate exclusivity to do it.

They're working on it right now, and I've enough experience with ads to tell you they're never going to make Google money off of their self-serve stuff. Why is too detailed for this, but suffice it to say their actions make me think they feel the same way, but also agree that their platform can make more.