There is now a Mastodon instance—mapstodon.space—for map and geospatial professionals and enthusiasts. If it had been up and running when I started The Map Room’s Mastodon account (previously), I might have signed up for it there. (Update Nov. 20: In the end, I’ve moved there: @maproomblog@mapstodon.space.)
It doesn’t matter that much which instance you sign up at (you can connect to any other Mastodon account on any other instance, unless your instance blocks that other instance, which happens when, for example, an instance is full of racist trolls), but instances have local feeds, which is nice when your instance is full of people who share your interests. I’ve already found several familiar faces and/or institutions at mapstodon.space.