Before the whole Reddit migration I was passively aware of the Fediverse, I thought I somewhat grasped the concept, and had created a Mastodon account, but never really used it.
But once the migration started, I decided to try out Kbin, and really liked it, and even started to use Mastodon more often.
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.