For example, I literally haven't seen some of my high school friends since we graduated 22 years ago, but I know all about their kids and families and their recent successes at their jobs. I like knowing that I'm still connected to people I spent literally every free hour with in my youth.
It's especially salient for me because Facebook didn't exist when we graduated. Many of them I've only reconnected with in the last 5 to 10 years, so I remember what it was like when we didn't have Facebook to keep in touch.
It has real value for me.
Why is that so useful though? If they were really important to you personally, you'd have kept in (active) touch with them. If you don't live in the same town as them, then you're not likely to encounter these people outside of a reunion.
I'm not trolling or criticizing, I'm genuinely curious why "staying in touch" in this way is so valued. I can see the value of LinkedIn (it's always possible you may develop business or professional relationships with passing acquaintances from your past and I take care to prune my connections occasionally) but the draw of FB is a bit mysterious to me.
Facebook to me feels like those xeroxed summary-of-my-fantastic-year letters that some people stick in their Christmas cards. Mildly interesting, maybe, but also kind of tacky and pretty impersonal and low effort.
The asynchronicity of the service is what enables me to keep in touch with so many friends.
The secondary role Facebook fills for me is as an address book with up to date contact info for all these people.
People you're actually close to matter 100x more, and you're going to stay in touch with those people regardless.
I have a Facebook account that I set up for some service that needed it for a login. But I've never used it beyond that.