The lemmy.world statistics are visible from the home page. There are currently 130,000 registered accounts on lemmy.world which is far larger than any other lemmy instance. Primarily because most other instances had restricted registrations of some sort for awhile.

I can’t think of any other website of a similar style that is larger than the entirety of lemmy. The next biggest might be kbin.

First question: Maybe someone will answer. The way the fediverse seems to work, that doesn’t really matter. A more interesting, maybe, question is how many people are using Lemmy with all the instances combined.

Second question: I’d be surprised if it isn’t.

If you’re new and looking for content, browse all and add the communities that interest you, or go to https://lemmyverse.net/communities and start searching up things that look interesting. A lot of the communities are still really low volume, but you can find many with a lot of action.

Remember, lemmy et al., is not a commercial/corpo run forum. It has no monetary incentive to grow exponentially every quarter. There’s no shareholders or BOD to bootlick. Let dot world grow organically. Sometimes it will even shrink but one day it may spurt.

https://fedidb.org/current-events/threadiverse might answer at least your first question, tho I have no idea how accurate those numbers really are so take all that information with a grain of salt.