Lemmy seems to have stabilized, if not stagnated. There are a few active political and tech communities, but they’re not particularly diverse in their viewpoints. Most of the other communities I used to follow on Reddit (local/regional discussion, US sports, cycling, etc) never really reached critical mass here and are basically ghost towns.
Mastodon is similar. I’ve observed a number of moderately high profile accounts move back to X/Twitter in recent weeks/months. I’ve been on Mastodon since 2017 or so. I think 2023 or 2024 were probably the high-water mark for activity. Things may look different from other perspectives.
I’m still here, and have no intention of going back to X/Twitter or Reddit any time soon, but the fediverse increasingly feels like usenet. Still inhabited by a few diehard lifers, but generally irrelevant to the broader population.