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).
While the user bases of Piefed and Mbin are smaller, they should be included as they federate with Lemmy and interact with our communities.
The same is true of Mastodon but that’s a lot harder to count since most Mastodon users are not interacting with Lemmy (since it’s a different style of platform - microblog vs link sharing).