I’ve been using Mastodon for a couple of years now and it’s become significantly more active in that time from what I see anecdotally. That said, I’d say the key question for health is whether the community is big enough to support ongoing development and hosting. I think at this point the answer to both questions is a definite yes. There are millions of users in the Fediverse now, plenty of users are technical and are actively contributing.
I think we’ll see active users fluctuate over time, but I don’t see the core base of users abandoning Fediverse at this point because they’ve already established their social networks here.