Makes me wonder just how many are active users

MAU means "monthly active users". As you can see, the ratio MAU/users is not higher for Mastodon than it is for Lemmy.

@mbryson @maegul
According to the CEO of Mastodon last time I chatted with him, the software tracks log-ins, so people like me, who do not log out and in again, would be invisible. So I would say with a certain level of confidence that those are active users you are seeing. Unless he changed the code of course, but it did not seem to be high on his ToDo list.

... and then went back to twitter, but did not deactivate their #Fediverse account.