I honestly don't know what they'll do or what aspects they' decentralize or centralize. I think for certain aspects it makes sense for security to put it on the decentralized ledger but for other purposes it makes more sense to put it on the centralized ledger (for legal compliance reasons).
So it totally depends. FB currency even if decentralized would also be capable of being whitelisted and blacklisted by the Facebook community standards. This means you can have a decentralized token which is also censored or heavily influenced. Any token which has interacted with anything illegal could simply not be accepted by Facebook, and can be tainted, blacklisted by Facebook, etc. They have ways if they choose to go decentralized.