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).
You’ll know when it hits mainstream when it’s referred to on the nightly news and every media outlet is running at least an account for each program, but hopefully an instance for each organisation.
We’re nowhere near that.