I moved to Mastodon. 1 day later I have a pretty nice feed and good amount of followers. Many of them old familiar now Twitter-refugees.

If we learned something from Tumblr, no one. The value was in the community, and just building a replacement doesn't recreate it.

ActivityPub has been around since 2018, Mastodon 2016.

If you mean something else, how would it differ?