This would make much more sense that having they credits. I imagine many people would join with bitcoin that wouldn't do so with the current system, just because it's weird to have credits that are only valid for one website.
But Facebook wants you to have credits stuck on their site. Just like businesses want you stuck with their gift cards, rather than cash that can be spent anywhere. It's guaranteed business.
exactly -- so it makes perfect sense IMO to both accept BTC for payment (targeting the geek/libertarian/bitcoin crowd), and also retain the "gift card" nature of their credit system.