I've noticed a huge spike in Mastodon usage of late, and it looks like this is becoming a self reinforcing phenomenon. It's starting to get more and more mainstream coverage, and that in turn helps legitimize it as the platform to use as the Twitter alternative.
It also helps that Mastodon is relatively mature now, and there aren't any commercial alternatives to Twitter that people can flock to right now. So, it's basically the only game in town.