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.
I myself have one kbin account and one lemmy.world account because kbin was not stable enough the first day and i did hear great things about the lemmy.world stability. I do use both though and also considering opening a feddit account