afaik the decrease in users has mostly been on mastodon.world, where the suspected reason is to a large degree requiring registration applications to prevent spam bots. this seems to scare off a lot of users unfortunately. i'm not involved in the mastodon stuff, but afaik there is currently no system similar to what we have on lemmy or piefed to automatically accept applications after certain checks were successful.

In total the fediverse is stable or slightly growing. ActivityPub is how fediverse instances exchang information. More and more platforms are getting stood up and there are also a bunch of instances of those platforms.

Platforms are things like Friendica, Lemmy, Mastodon, PeerTube, Funkwhale, Hubzilla, Misskey, and now Pifed. Instances are things like lemmy.world and sh.itjust.works.

This approach was intentional. No single platform or service controls everything and if someone starts misbehaving it's easy enough to go elsewhere.

That's not to say there aren't downaides. We have a ton of communities for similar topics on different instances and/or platforms. Each of these communities need to be moderated, have an active user base, etc.