You *do* need Facebook to log in to various other services. I used it, until recently, for Tinder, for example.
This doesn't mean you actually have to post stuff on there, or do anything substantive with it besides use it as a login service. The only things I use my FB account for are logins (and not even that at the moment; I have a girlfriend now so I don't log into Tinder now), and being "friends" with a few distant friends and not-so-distant family members who insist on using it. I never post anything.