I don’t understand either. Usually someone responds with something like “the local <whatever> only posts updates on <app> so what choice do I have” or something to do with peer pressure or FOMO. I don’t doubt that people deal with those problems but they seem like better ones to have. But it seems like a lot of people just cannot imagine living without the apps anymore.
Facebook is intentionally designed to be as addictive as possible. It's not just FOMO, https://news.ycombinator.com/item?id=24579498