The application GroupCard is currently testing the new payment system live for all accounts that have it installed. I included some screenshots below. It’s very straightforward: There’s a big “Pay With Facebook” button, similar to the “Facebook Connect” buttons you see throughout the web. Next to that, there are the other options to pay with Visa, Mastercard, etc.
Clicking on the “Pay With Facebook” button pops open an overlay which asks you to confirm payment via your Facebook Credits. My $2.99 card cost me 30 Facebook Credits. Expect to see this roll out to other applications soon.