For various reasons, Diaspora* seems quite prone to this. I have a number of suspicions as to why, though stagnent development and either intentional or incidental decisions which have made the platform almost perfectly opaque to general web search seem to be likely factors.
Mastodon, by contrast, seems to acquire a new major group of new adopters every year or two, that I've noticed.
(I'm active on both platforms.)
Could you give an example?
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.