If we assume x number of events per day on the network, the more relays, the fewer events each relay needs to store. Text events are tiny, you can fit all of wikipedia's text on a DVD. If we integrate with other P2P networks like #IPFS, #Freenet, #Hyphanet, etc for storage of media, then space isn't much of a concern. Tor isn't needed, NAT punching protocols exist and are mature, plus IPv6. 

Think of it this way: Can your phone store all the text from all your social media posts? What about all of your 10 closest friends? Yes? Then this can scale.

But you don't need to take my word for it, look at existing P2P technologies which have been doing this kind of routing for years. There's no need to centralize around relays.