Here's the relevant excerpt from the updated prospectus:
For example, in 2011, approximately 15 million users purchased virtual goods using Facebook Payments. If the Platform apps that currently generate revenue fail to grow or maintain their users and engagement, if Platform developers do not continue to introduce new apps that attract users and create engagement, if Platform developers reduce their advertising on Facebook, if we fail to maintain good relationships with Platform developers or attract new developers, or if Platform apps outside of social games do not gain popularity and generate significant revenue, our financial performance and ability to grow revenue could be adversely affected.
Facebook Credits launched as an alpha in May 2009. The beta stage started in February 2010 and ended with a final version in January 2011. As of July 2011, all Facebook game developers have been required to only process payments through Facebook Credits. It is not (yet?) a mandatory payment option for Facebook apps.
Facebook takes a 30 percent cut of all revenue earned through Facebook Credits, leaving developers with the remaining 70 percent. Some believe this violates U.S. antitrust laws. Facebook Credits is growing as a percentage of Facebook's overall revenue, but the majority still comes from ads.
If Facebook can figure out how to get more users onboard with Facebook Credits, the virtual currency will be a goldmine for the company. It's no wonder the company is including the payment infrastructure as part of the Facebook Platform for Mobile.