Everyone keeps focusing on Facebook as an ad platform, and pointing to the difficulty in monetizing mobile via ads. But I'm betting Facebook is on the brink of rolling out a digital wallet. They already act as an authentication/identity service with hundreds of millions of users. If those users linked their FB accounts to their bank accounts, they'd be able to buy anything on any integrated website that they log into…
The problem here is would you trust Facebook with your wallet? I wouldn't, and I'm betting most wouldn't after all of Facebook's Privacy gaffes.
Getting into payment processing will expose them to SOX and PCI compliance. They'll still be able to reset your privacy settings for your profile, but they'd be sued out of business if they did something like that for your payment profile. No matter how much it looks like they do, Facebook rarely makes mistakes.