It will certainly vary by Twitter community, but I'll say that the infosec community on Twitter has certainly either moved to Mastodon exclusively, or is posting primarily on Mastodon and cross posting to Twitter. Additionally I'm seeing more and more of writer Twitter move to Mastodon, including several large authors leaving Twitter entirely. I expect that politicians in many forms will be the last to move, but journalists are likely to have to leave Twitter entirely given Elon's current anti-journalist tendencies.

I'm old enough to have seen the "end" of Myspace before... and kinda like is happening with Twitter now, it started with a little bit at a time, then all at once almost everyone was gone.