There was another group that showed up ~July/August 2018. Notable amongst those was the actor Wil Wheaton, who discovered Mastodon had trolls too, and left shortly after. (Mastodon's anti-abuse practices and tools evolved as a result. Sorry, Wil.)

There was a group of Indian users who came aboard following oppression of opposition groups in that country, a few years ago.

Musk's Twitter bid has initiated yet another.

There may be others, and there seems to be a general secular growth trend.

Retention amongst new adopters varies --- there's definitely a fall-off curve. But on balance, the trend for now seems to be growth, and the result is an evolution of participants rather than a static set.