Facebook Credits make up Facebook's virtual currency; the currency became available to some users last spring. Those users could buy gifts with it. Facebook then made a deal that gave users the ability to purchase Facebook Credits with their PayPal accounts and offered Facebook Credits as a currency option to several application developers, including uber-huge game-makers Playfish and Zynga.
Facebook says it's taking the 30% cut so it can invest "heavily in the ecosystem" by educating users and marketing to them about the currency, testing out incentives to get people to try the credits out, and seeding credits to get people comfortable with them.
Here's an interesting coincidence: That's the same percentage that Apple takes from App Store sales.
Just before 2010 began, we named virtual currency as one of the major moves to expect from Facebook this year. That was after we predicted earlier in 2009 that Facebook would deploy a currency.