ActivityPub was not invented in 2018, in fact it has a history of multiple protocol iterations since 2008 including OpenMicroBlogging and OStatus.
2010 another company named diaspora raised $200k to create another federated protocol. In 2018 #diaspora peaked at 1 Mio. users with 200 federated servers. Then the core dev decided to ignore #ActivityPub. That was the end.
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To be fair Nostr is innovative when it comes to using servers only as relays to prevent the moderation drama that is fragmenting the Fediverse now
P.S. missing so much Diaspora's ability to publish posts for lists of accounts. Mastodon's everything-is-public is just stupid
