You could have another unified micropayments platform, but that's still another system I need an account for, etc. (unless you can back-end to iTunes...good luck! :)
The facebook one could takeoff, but then I think you will only be able to offer an app for free and sell virtual goods. Still the same kind of profit margin for facebook as apple though.
Would be lower since they only handle payments, and don't have to manage distribution, infrastructure and a bunch of services around the product itself, not just its sale and billing.
