Having kids it's apparent that the whole 'look at me' idea is a fundamental part of being human. It's like quantum mechanics, things don't happen when not witnessed. Social media is the perfect place to replace parental acknowledgement later in life.
It's no different with games, high scores, achievements, autolog, game dvr/streaming, all tie into the fundamental need to be acknowledged. We're herd animals, we're defined by the opinions of and comparisons to the rest of the herd.
I don't use Facebook anymore, I guess I get my acknowledgement through games. Plus already spent to much time on this pointless site :) Ooh a blue icon in my buddy bar...
Facebook has it's practical uses too. My wife hardly ever uses the phone anymore. On Facebook she'll have multiple conversation windows open at once while working or playing a game. You can keep track of 4 completely different conversations through chat windows with ease, try that with a phone.
The other big thing of Facebook is, it replaces small talk, the need for which Terry Pratchet explains best
He also uses the nomes as a group on the outside looking in on our own culture. One such instance I loved was a conversation between Masklin and the Thing. He asks the Thing to translate what the humans above him are saying, and he says, "The man with the sign is here to take our human to a hotel. It's a place where humans sleep and are fed. All the rest of it was just the things humans say to each other to make sure that they're still alive." Of course, Masklin asks him what he means by this, and he is told "Things like 'How are you?' and 'Have a nice day' and 'What do you think of the weather, then?' what these sounds mean is: I am alive and so are you.