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. There are currently almost 200k total users spread across the top 10 non-bot lemmy instances.
We're really building something here!
EDIT: Looking for a lemmy app? Here's a whole list: https://lemmy.world/post/465785
I think you're right insofar as onboarding is concerned. Once you've registered, though, Lemmy is relatively straightforward to use. Changing your user settings to display posts from ALL federated Lemmy instances on your front page helps with discoverability. That should be the default setting, but it isn't. That setting is associated with the "Type" parameter (found just below "Theme"). It isn't terribly obvious.
Before that setting becomes default, the "Hot" algorithm needs to get a major overhaul. It keeps spamming the top of my front page with posts that have zero comments and around ~1-10 upvotes as the results from federated instances start trickling in.