Facebook login became the universal login system. Plus they have a system for buying credits which they use for making transactions such as buying games and other items.
It would be interesting to expand this mechanism into a 'Social Wallet' for Social Payments. If a merchant has a facebook login than it can be recognized it in a trusted way. In addition, it would be possible to generate a unique number for each transaction in a way similar to what Paypal is doing today. In this scenario, there will be a set of merchants selling items in real life and using facebook as a payment system. The payment is nothing more but a facebook credit transfer (again similar to Paypal). Likewise, they can use Facebook to transfer social gifts, game points, and other types of virtual objects.
The real power of this is that the system will be available for 700millions of users and it would work perfectly for micro-payments. Plus facebook is available on many different devices so it would possible to pay on phones, tablets, and PCs.