I mean, I think it probably depends on your use case, but yeah, it is, at the very least, dying. I haven't felt the need to use my dormant account for just under two years (with the latest AI nonsense I should probably just delete it; I'd been holding off until now because, well, it's 18 years old, and I have some sentimental attachment).

Mastodon's been a largely acceptable replacement.

Indeed, network effects are durable but not unbreakable. I haven’t deleted my Twitter account yet, mostly because there are those (increasingly rare) occasions when it’s necessary to log in to view some thread or the like, but when I find myself there, it always feels like a raging dumpster fire. Building a new network on BlueSky took a while, but it was definitely worthwhile, and while there are a handful of people I miss from Twitter, I have a good community that fills the need that Twitter used to. I suspect that for most tech people, Mastodon is going to be the new destination and there will never be a single destination for social networking again, but maybe that’s a good thing.