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.
That's something for Facebook (the company) to fear rather than hold dear, I think.
I too was 'logged off except' a wealth of university-related groups. Then I was 'logged off during' writing up my dissertation, and finally just 'logged off'.
(And now I'm still contributing to user count, while continually thinking I should just log in to delete my account, as I've heard a couple of times from people that thought I was 'on Facebook' or 'what happened to your profile' and maybe more people that think I'm just ignoring them than I'm aware of.)
