Facebook is all about friends, so it stands to reason that friends be the main facilitators of marketing through the social network. This is a sentiment driven home by Facebook's new Sponsored Stories, as well as another new product they're testing called "Buy With Friends".
Buy With Friends, revealed by Facebook at the Social Apps Conference, is a way for Facebook users to let their friends know about virtual goods purchases they make, and a lot of the speculation regarding this product pertains to the possibility in expanding it into physical goods and services. In other words - a Groupon competitor.
With Buy With Friends, users can unlock deals and share them with friends - encouraging other users to take advantage of the same deals. It's not hard to see where this could be attractive to businesses in a Groupon-like sense.
Facebook already has Facebook Deals, which users can use to check into businesses and get deals for doing so. It's not inconceivable that these two products could merge somewhere down the line, especially when you consider how big this space is getting (Google Offers should be appearing in the near future).
Keep in mind that Facebook just acquired mobile advertising company Rel8tion and is now making Facebook Credits acceptance mandatory in all Facebook apps. As the mobile payments space also heats up this year (with offerings expected from both Google and Apple, to name a couple), it would also not be surprising to see Facebook Credits become an option for paying for physical goods and services out in the real world.
The company is already closest to owning the Internet user's identity (a concept being pushed by even the government now), with Facebook log-in available all over the web, my guess is that we'll see a lot more Facebook Credit purchases around the web even sooner.
Let's also keep in mind that Facebook is already in the stickers-for-businesses game. Last year, they handed out stickers to businesses for getting "liked" on Facebook. Is it that big a stretch to see them offer "Pay with Facebook" stickers?