When I first signed up, it was mostly LGBTQ and furry. That is, people who left Twitter looking for a safe space. And then Twitter made the already infamous @reply change, and the floodgates opened. Just the flagship instance has increased its userbase by 20% in the space of 24 hours or so.
It will be interesting to see if adoption reaches critical mass. Right now all the conversation on masto is about masto, which doesn’t exactly build a community.
If you’re interested, I’m gamehawk.
Update: https://instances.mastodon.xyz/ has a running list of instances, since the main instance is kind of swamped. In hindsight, Eugen should have made a separate homepage/site for the federation than for the flagship instance.