My first impression when I saw this news was that this would be about as 'decentralised' as Tor is. Facebook (and pals) will always maintain significant enough control over the nodes in the network to both maintain consensus in the blockchain, and also to subvert whatever consumer-friendly guarantees they'll claim to make. No different to Tor and US spy agencies controlling enough exit nodes to defeat the purpose of Tor.

Facebook and privacy are fundamentally opposed, so based on known behaviour the currency itself is most likely a hook into more of its users' lives.