Instead, I could see this taking a similar direction as web authentication: instead of "Login with Google+" or "Login with Facebook" buttons, you have "Send $ to Google Wallet" in your FB-pay interface.
I'd also point out that Facebook, by virtue of having a ton of data about their users, is well positioned to do things like identity assertion and fraud analytics; two of the more critical aspects of digital payment. Makes sense to me that they join the fray.