Honest question, why not just go with diaspora*[0], which seems to run relatively smoothly and already has a (small) user base? Is there functionality they do not posess or are there issues with their architecture?
Now this may be old, but it had something called "pods" which I think are servers. And you have to run one, or register with one. Once done it would store all the data. There didn't seem to be a way to have intra-pod communications. This didn't feel sufficiently decentralized for me. Basically it seems more like a tool to create your own facebook. I still have to trust the guy running the pod.
I was expecting a locally running app, with all my stuff available to it, one or more bitcoin style IDs and external servers acting as a napster style introductory service, which once complete would have nothing to do with the encrypted p2p communications that followed.
I think with a sufficiently scoped out messaging protocol and simple demo client software something new and important could be created.
Since everything would be encrypted for the end-recipient it may well be data intensive, if sharing videos amongst many people for example, but perhaps some shared secret/ one-time pad/ dropbox (old and new meanings) would be a good solution to this.
[0] https://diasporafoundation.org/