A lot more common than logging off permanently is logging off except. I've logged off Facebook, except there are some Groups I'd be sad to lose touch with. For some people, they're off Facebook, except there are people they only contact through Messenger. Some are off Facebook except they like to see what Grandma says. As long as there's an 'except', Facebook still has that user.

> Are general social networks actually useful though

I'm not a big Facebook user, but my parents have been caught up in the bush fires currently raging in Australia and got cut off from power and internet (no power for satellite internet, and the mobile mast ran out of backup power), and finding a local Facebook group created to discuss the fires was fantastically helpful, I got in touch with their rural route mailman and people in the local village and found a bunch of good local news sources.

In comparison there was nearly nothing on Twitter (one Sydney reporter who was tweeting from her dispatch for two or three days). Main-stream news sites were utterly useless with no detail in their reports. Without Facebook I would have had drastically less information and more worry.