The service builds on the gift services in Facebook.
These gift services have been tracked by Lightspeed Ventures for a while now and estimated to be worth around $15 million a year back in January 2008, increasing to $35 million by September 2008 and probably upwards of $60 million today.
The credit-based system is described in more depth in Venture Beat:
and what it implies is that Facebook has launched a first phase, virtual currency for swapping appreciation in virtual units amongst their now almost 200 million users.
Cashing in those credits for trading in real money?
Can't be far away … oh yea, that's Spare Change isn't it?