Social Networking

Help me out people... why do people get on social networking sites and only talk to people that they already know and nobody else? What the hell is the point of that? If I wanted to talk to people I already know, why am I on the social networking site in the first place? I'm sure if I know these people in real life I can easily get in contact with these people via phone or text, right? Or maybe I just missed the boat. Maybe, I'm the only one that thinks you are supposed to network with people that you don't know b/c "hey maybe we can help each other out". Nah, I think I'm probably on the wrong boat and you are only supposed to just talk to people you know...