That's sort of a misreading of the figures though, isn't it? It's not that those 950K sign-ups have all disappeared; you just have a slightly higher replacement level of new entrants. What it describes have a fairly consistent monthly usage rate -- not leaping up, but not dropping either. That also seems to be what the more in-depth statistics here indicate: https://the-federation.info/mastodon
I'm not saying that you're any more wrong than the idea that federation is currently "in vogue", but it doesn't seem right to suggest that this is something that is in decline, either.
Maybe you could argue that in the face of increasing numbers of adoption of social media more generally? Are we steady-stating there too, though?
That's not a cultural wave, that's a fad more swingy than gym memberships in January. "Vogue" technologies don't have a 99.93% churn rate.