Well, it's plausible, but the most natural case is that acquaintances use Facebook as a low-touch way of staying in contact. I admit if I wanted to be friendly

Well, it's plausible, but the most natural case is that acquaintances use Facebook as a low-touch way of staying in contact. I admit if I wanted to be friendly acquaintances with people, then Facebook is probably the best way to do so. However, close friends share information without using Facebook. I call my close friends on their birthdays. My acquaintances, I'll +1 the long thread of "happy birthday" messages on facebook, just like everyone else. tl;dr: Facebook is unnecessary but useful.