When developers go to convert Credits into cash, Facebook gets 20 percent of the transaction amount. Credits are used to purchase items such as movie views, or tickets to concerts. In principle, that is analogous to Apple taking a 30-percent slice of all retail sales from iTunes.
But there are other angles. Facebook also allows users to exchange gift cards for Facebook Credits through Plastic Jungle. The value of the gift card, less a fee of about eight percent, gets translated into Credits added to a user's account.
Likewise, Shopkick allows users to earn "kickbucks" just for walking into participating shopkick retailers like Best Buy, Macy's, Sports Authority, American Eagle and SIMON Malls and turn them into Facebook Credits. So far, with the exception of the in-app purchases program, the other programs are ways to turn "real" currency into Facebook Credits. Only developers can actually export Facebook Credits back out to the real world as real world currency. But you get the point: there is some potential here for blending virtual and real currencies more seamlessly.