Almost! At the time of this post, lemmy.world has a whopping 25733 users and is growing fast.
Since my last post yesterday, it has added 4000 new users, making it the clear second-largest lemmy instance out there. Also quickly catching up to lemmy.ml's 36000 (not taking new signups).
beehaw.org (3rd largest) sits at 12500 users, partly because of more restrictive registration requirements.
While the number of registered accounts is interesting the monthly active accounts numbers are more interesting as lemmy.world is claiming 8,005 while lemmy.ml is claiming 3,467 and beehaw.org is claiming 2,919. lemmy.world's growth in posts also indicates that it is the most active of the lemmy instances.
Lemmy.world is extremely new, I don't think it is more than a few weeks old but the person who created it is very active and working to improve it constantly. I think that shows in its uptime vs the other instances.
Here is the oldest post by ruud, the admin 2 weeks old: