Facebook's business model is busted. Costs are higher than revenue per user. The model has to change, become a search gateway, start selling products people want to buy, or cut costs features and charge high bandwidth users.
No facebook game will ever get a payment from me while they're relying on third party providers. There's a slim chance they'll get my money through paypal (since I've had a paypal account longer than I've had a facebook account), but as the money you pay goes into solely one game there's no interoperability. Every developer is stuck developing their own way to implement credits using companies that have no reason to care.
People are going to trust facebook, but they're not going to trust 'cherry credits' or whatever suckass companies you know nothing about that you're signing your credit card information over to. Facebook is already trusted by its users (or they wouldn't be users), which gives them an unbelievable edge without them having to play unfairly against the other game credit providers.