Lemmy.world grew from about 51k users when third-party reddit apps started to shut down to about 84.8k users at the time of this post.
Definitely felt some growing pains in the past few days, but it's great to see the platform more active now that things have become more stable.
So, welcome reddit expats!
Ruud also runs Mastodon.world which has 160k+ users and manages to survive on donations alone. Not sure where the cutoff point is for when that is no longer viable, if there is one. Mastodon.social is huge and takes sponsorships and also gets some grant money I believe. They don't run ads per se though and claim all sponsors accept contracts stating Mastodon is not going to be run in a way that is influenced by sponsors.