They already exist and are called facebook credits: http://www.facebook.com/help/?page=1038
They hadn't truly launched the credit system in a real sense. It existed, but was so low profile as to be non-existent, which I assumed was intentional. But with Farmville signing on, that's a pretty real launch.
The fee structure is still exhorbitant, though. Again, maybe intentional, to prevent it from being a practical parallel currency.