My point is that I think the narratives we construct around online fan communities/fannish social networks and fandom migration tend to characterise certain websites as the be-all and end-all of fannish activity [...] and I can't relate to that, but I have no idea if that's just me

Definitely not just you!

Because I am Extremely Old™️ I've always kept my online presence centralized around websites I own. Like, I will crosspost content to where the community presences are at (ref. e.g. here), but I treat it as a transient interaction and always have an archive/home base I own. So it's not such a big deal when third-party sites and communities come and go.

It wasn't necessarily a conscious choice to do things like that, though. I just... always have. And happened to luck into it working out pretty well (lol).