>I believe there is room for a private social network that is peer-to-peer, encrypted, with no central servers and an open spec.

We already have this. It's called "Diaspora". It's a great idea using all the things you talk about: decentralized, peer-to-peer, etc.

No one uses it.

>but I don't require mainstream adoption to use it.

Then I encourage you to set up your own Diaspora account. Maybe if enough people do so, it'll eventually get somewhere.