Facebook founder and CEO Mark Zuckerberg did an interview with InsideFacebook talking about the Facebook ecosystem. One thing touched upon was Facebook Credits and it not being a major profit driver for Facebook despite a 30 % fee for developers.
One reason is that Facebook Credits can be bought with premium text messages, which cost far more to process than credit cards or Paypal.
Facebook is using Zong for mobile payments, and it's worth noticing that Zong has done some really, really nice business development with their Zong+ product.
Basically, after a purchase of credits or virtual goods a user is asked if she wants to connect her credit card to her mobile phone number (in return for additional credits/virtual goods). The result is mobile-initiated credit card payments and the combination of easy to purchase (mobile) and a low-cost funding source (credit cards). Probably the smartest thing I've seen in online payments in the last year.