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.
Source: https://lemmy.fediverse.observer/list
Exciting to see all this growth!
Yes. Beehaw admins have just blocked access to their instance for lemmy.world users. In order to get the most content possible, it's best to sign with an instance that isn't blocked by other instances. If you don't care about that then it doesn't matter who you sign up with though.
Did they say why they did that?
The rationale appears to be that they don't have the moderation tools necessary to deal with an unvetted userbase. Lemmy.world doesn't require any prior user verification, so anyone can join (just like reddit). Beehaw claims that they were receiving a large amount of trolls or other negative traffic from lemmy.world and shitjustworks, so they have temporarily defederated these instances.
This may change if better modding tools come around.