I joined Mastodon a couple of times in 2018 but could never make it stick. Coincidentally rejoined last year just before the Twitter implosion, and maybe it was all the new people I was able to start using it in a way that made sense to me and rather enjoyed it, despite not using Twitter.
As soon as the writing was on the wall about the API changes, I created an account on Lemmy and here. That was end of May and the big migration happened end of June.
So tl;dr, I've been aware of it since 2018, but didn't actually start using it until end of 2022.