Mastodon has, from a google search a few days ago about 975k users.
Bluesky, goohle search 30 seconds later had 20 million users, and a news story that they gained 700k users in the past 24 hours (at the time). So I'm unclear if the 700k is included in the 20 mil, or if it should be more like 20.7mil.
Seriously, Mastodon has languished under Gagron and it's probably not gonna grow much more. They spent years hemming and hawing about adding quote posts, they ignored the problems with onboarding for years before just defaulting to .social, they're still clueless what to do about moderation and harassment despite piles of user feedback, bug reports, pull requests. This is one case where an open source BDFL just entirely tanks a project.