@staab wrote this: https://github.com/nostr-protocol/nips/issues/75

“Nostr's gossip protocol is too naive to scale. Relays will become either too big to run cheaply, or they will lack the reach to be useful to end-users. If relays can mirror content more selectively, and along natural borders in the social graph, it will help to shape the network topology to allow small relays to host only content relevant to its users.”