At the time I knew what the fediverse was and I was using Mastodon quite consistently, and I was waiting for a federated alternative to reddit. Raddle at the time was already a thing, but federation was not on their roadmap.
As per how I found it... here's a comment from four years ago :)
Longer than I thought. When I tried to register I was annoyed that the username was taken, but it turned out they also dropped the info in my password manager back in 2021!
My very first account was in Feb of 2022, but i kinda abandoned it until the 3rd party apps blackout on June 12. Then i came back and really started to kick the tires.
December of 2022. I was moving away from closed-source social media, and after Twitter -> Mastodon, Reddit was next. I didn't become active til the whole API thing tho, because Lemmy didn't have some communities that I liked. So I made one lol
I knew about Lemmy for nearly 2 years prior to The Blackout and slowly gained some traction after the birth of the instance I created my account in (which started as a subreddit but for reasons around one year ago their exodus began)