Lemmy active users grew by an astounding 1600% in June
From 2,997 active users across all lemmy instances at the beginning of June, the number increased to 52,797 by June 30th. Source.
An active user on Lemmy is "someone who has posted or commented on our instance or community within the last given time frame.” Source. That means lurkers are not counted as active users.
Server costs and some instances (like beehaw.org) are particular about who they let in. That's why I just refer people to lemmy.world since it's the easiest instance to sign up with a lot of local content already.