You would encrypt the data before giving it to a rogue server using a key that reflects the access you want to permit.

So if you want your friends to be able to use it then you'll need a shared key with your friends.

But only the clients have the key, so the servers pushing the data around can't access it.

Edit: Check out how Keybase handles this.

> would that keep a rogue server admin from accessing sensitive user data?

There are no servers, it is peer to peer. The pubs are just easy to connect to to bootstrap new users into the social network and have been mostly closed off now that it has become more popular to avoid bad actors from abusing that easy way in.

You can bootstrap on a LAN / WAN without pubs.