Number of (active) Lemmy users seems to stabilize and I think this is a great thing. Indeed we got a lot of users when reddit shutdown its API (I was among them despite being a long time oss user), many have left, but the community seems now to stabilize to ~ ½ of the big grow in june '23. I think this is very nice for lemmy, we can be proud of this project.
The stats come from: https://fedidb.org/software/lemmy
So all lemmy servers can communicate with eachother, unless a server decided to “defederate” with another which cuts the connection.
Futhermore, mastodon and other fediverse projects can also communicate with lemmy and back, but since the format of the posts and comments is a bit different this might take some work for the devs to get working completely correctly.