I use Nostr (obviously), but do not think it has reached critical mass yet. Content is too Bitcoin centered (saying this as a Bitcoiner). I think, Mastodon is a valid complement, though I do not get the focus on moderation.
Discussion
I have not messed around on Mastodon. I’ve had success getting ideas beyond just bitcoin by following more and more people. Agree it’s notes at critical mass but I’m excited to see Nostr grow.