In the beginning, Facebook Credits were just used for gifting friends virtual gifts over Facebook, and more recently, CrowdStar's Happy Islands launched using credits as the only way to buy virtual items for the game, and others, such as Restaurant City, Happy Aquarium and Tiki Farm, have also included Credits as a way to pay.
To get a better idea of how far your dollar with stretch with Facebook Credits, InsideFacebook printed the exchange rates for the virtual currency. One US Dollar is worth 10 Credits, which is not that far off from, say, Playfish Cash, where $1 is worth 6.5 PFC.
The nice thing about Facebook Credits is that it'll be easier to tell how your real-life money translates into virtual money -- each Facebook credit is worth a dime. Three cheers for simplicity.