I knew of it because of the minor migration to mastodon, plus the website for EndeavourOS, which I use for my operating system has a mastodon account for posting news about it. But like many other I’m not a big fan of the microblogging concept. I didn’t know about lemmy until the migration was getting ready to happen from reddirt (spelling intentional).
I did know about the fediverse and how it works in principal before then.