Peter Vogel explains why Facebook ended its Credits experiment:
Ironically, it’s the enormous potential of Payments as a revenue source that is causing Facebook to phase out the Credits currency. Payments as a revenue source is too important to Facebook’s future to take the risk of promoting an untested and unproven currency. To establish Facebook Credits, Facebook would have had to spend significant resources educating the public and building the brand of Credits. It’s a much easier solution to simply transact in an already established currency that users understand and utilize.