It's really just that network effect is a feature, especially on a communication platform.
Personally, somewhere on my project backlog is a plan to set up a personal Matrix server for this exact reason, as most of the public ones I've found stop just short of actually supporting bridging for any Meta platforms yet.
(And even if they did, E2E bridging is still WIP at the spec level anyway if memory serves, which would mean still having to roll my own for the time being... Because naturally, if I'm concerned enough about my dependence on other people's cloud services, or their harvesting of my data, to consider going to lengths like these, then I'm obviously not just going to just wantonly funnel a collection of DMs spanning about half my lifetime through a third party's servers.)
Facebook has survived long enough that people are likely to stay just because they're so accustomed to using it. The marketplace is great and some people have tons of family photos on the site.
AOL tried to be all things to internet users and wanted to be a walled garden at a time when cheaper alternatives were coming out that gave users more freedom. Facebook is resigned to the fact that it isn't a walled garden, but it does things to try to retain and gain new customers anyway.
Connectivity to your social circles is something Facebook provides that nobody else provides. There are other ways of connecting social circles, but due to network effects, they're all worse.
There's no site on the planet other than Facebook that lets me interact with my entire family all at once in a way that I know will reach them.
I'm not a facebook user but I did set up a google+ account while it was around and i did like that circles paradigm they had. For whatever else was wrong with that thing to make it crash and burn so hard and so quickly (other than google's corporate adhd and constantly being in 3rd or 4th place).. I did like that concept of circles as ways to target your content.
My kid’s class parent’s group is a Facebook group. In other places it’s a WhatsApp group. The only reason is everybody has a Facebook account.
I have one too, I just don’t log in anymore, so I have no idea what the other parents are talking about behind the scenes. This is the tyranny of a walled garden social network.
Advertising revenue is obviously more fragile, but I don't think Facebook cresting the high water mark says all that much about the tail trajectory.