These are perfect examples of the good that came out of the "everyone in one big public space" aspect of Twitter!

I've found the same thing -- I'm very active on Mastodon, but easily 95% of the people I interact with there are new weak-tie online acquaintances; most of the ones I'd enjoyed for years on Twitter haven't come over to Mastodon ... or if they have, they haven't found me and I haven't found them, because Mastodon makes discovery quite hard