Facebook credits are being phased out. Don't cry too hard.
Facebook credits, Facebook’s virtual currency, are slowly being phased out of Facebook. Facebook credits were first introduced for use as an alternative to real currency in apps and games. However, developers have largely shunned it in favor of their own forms of virtual currency or real currency, making Facebook currency useless.
With Facebook credits being thrown to the wayside, Facebook’s allowing developers to charge subscription fees for games, starting in July.
If you have a Facebook credit, they can be converted into local currency for use in Facebook apps and games.