Many social groups use Facebook to organize on. If an individual weren't on Facebook, they'd be much more socially isolated in the same way that if they didn't use a phone (considered a public utility), they'd also be more isolated. I suspect for some people, Facebook/Messenger/Whatsapp is more important than a telephone number.
Some services now require Facebook to use their service. For example, for people who can't afford to own a car, GetAround lets you rent neighbors cars, but they don't accept a government issued ID, only Facebook.