People complaining about Facebook on Facebook is profitable for Facebook

Delete it, stop being part of the reason other people are there. If you can realise it’s a cesspool and get out, so could they.

I encounter a surprisingly large number of people who seem ashamed about their Facebook usage and say things like “but I need it for […]” or “but I only use Messenger” or some other way they rationalise it to themselves

Just get out.