What does having a Facebook account have to do with being willing to use a virtual currency? I'm willing to use virtual currency and have a Facebook account, but I'll never use Facebook's credit system. I don't want Facebook to know what I'm buying.
Why not? People already trust FB with all their intimate details and relationships. I don't trust them either, but I'm pretty sure plenty of other people would.
Of 500M+ users even if a tiny fraction adopted it, it would become a huge deal. I don't know what the stats are for virtual currency adoption rates, but even the most widely adopted currency probably wouldn't match what the entire Facebook ecosystem would bring in