drew up a diagram of how ActivityPub (the Fediverse) and Nostr works and how Mostr bridges them, and how non-spammy it is https://void.cat/d/AkrQm1XmBMuWApGZUBNG6c.webp
Discussion
if you took the frontend (client) of an ActivityPub server and modified it to talk directly to ActivityPub relays instead, you'd have something VERY close in concept to what Nostr does. Nostr basically eliminates instances. users sign off their messages with cryptographic keys instead and belong to no particular server.