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 think this is the key to natural growth. Especially as more mainstream sites move to login required, and more and more search results are AI slop. Lemmy could be the last open and search indexable platforms left soon