True, we need ways to extend beyond the bubble. I think we might be able to store 3rd degree contacts even on mobile browser clients. Native clients or at least relays can store up to 4th degree, which I believe covers most people on the internet. Also one simple thing that can help is showing posts from unknown users that your social graph reacted to. Completely unconnected users are tricky, I can't see a spam-proof way to accept messages from them besides zaps.