I thought about using smartphones for this kind of thing. They are always online, like a web-server. Also, everyone can install an app.

A server which runs on a smartphone and holds all your data and only friends and services you like, get that data.

The biggest problem would be traffic, I guess, but it could probably be minimized with sending only deltas and p2p-meshs.

This system could use a web-api to integrate better into current landscape.