My answer to most of your questions (which are good ones!) is "go look at the activitypub/ostatus world, especially Mastodon". Federation between smaller, user-supported social sites creates a very different world than ones built to put eyeballs in front of ads.

I may be biased by having pretty much ditched Twitter for a Mastodon instance that I run for myself, a few friends, and whoever else thinks it would be cool to be @username@dragon.style and have a Twitter-like thing that lets them emit 7k tweets if they want to.