Not sure what decentralised means. Do the different hosted versions communicate together and sync users and data across each other? (like the blockchain etc. Because that would be fairly important).. Also a real sneaky pro move would be initially to allow people to "sign up" via Facebook :) - once they're registered and the network has their email address then Facebook can become useless, just lowers the barrier to entry and its ironic that Facebook can be used to replace itself.