The same problem we have with other networks of trust: Signing works alright if you already know the people you expect messages from (but in that case, it's als

The same problem we have with other networks of trust: Signing works alright if you already know the people you expect messages from (but in that case, it's also no better than the invite-only social networks the OP talked about). However, the real problem is getting to know new people or vetting messages from people you don't know. In the future the OP sketched, you can never be sure that an interesting new person isn't actually a bot. Knowing the public key of that person won't solve that problem.