I encourage you to read Eric Eldon's piece on InsideFacebook about how Facebook's most popular game, Farmville, now defaults to Facebook Credits and Payments. As location and geo dominate the blog headlines (Twitter, Foursquare, Gowalla), Facebook Credits has managed to stay relatively under the radar... but Facebook's payments platform is important as it is going to be very big. And, despite being young, Facebook Credits is now exposed to Farmville's 84,000,000 monthly actives... that's quite the launchpad.
