There used to be the Alexa ranking for websites but it seems to no exist anymore. Elsewhere I only could find for lemmy.world ranking (something like 55000, yay!) but not the actual userbase, and nothing about any other instance (let alone the entire Fediverse).
I mean you say it’s stagnant and that may be technically true, but I see plenty of posts per day without repeats. I like where we’re at.