nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqcgd35mxmy37vhkfcmjckk9dylguz6q8l67cj6h9m45tj5rx569cqry82a3 Ok. Say NOSTR grows to 2 billion users, about the size of most Meta owned platforms.

How do you ensure that all of them, without either forcing them to use a centralized list of trusted relays, or making them add so many relays the client can't handle it, can contact each other?

(The answer is, you don't. There's no way to do it with the relay system that's currently in place. You'll have to force a connection to a central relay.)