I joined a while back (after the rise of Mastodon following some edgelord's purchase of Twitter, although I had heard about both that and Lemmy long before), and I still probably won't be active on Lemmy until Reddit dies off; I joined this instance because it's the most relevant (nothing akin to Mathstodon, where I have the same username) and I was advised against just flocking to the main one.

I was most active on /r/learnmath and /r/Buttcoin and I might go to refugee communities related to both, and to things like /r/learnjavascript (where I was active until a few years ago, also as an answerer of questions) and /r/badmathematics.