Social network: Do you "Like" it?

If social networks such as facebook truely represent a virtual place for people to share news with their friends, relatives and acquaintances, why is it so rare that you see someone posting some sad news? Though it's true that it is a human tendency to rather share good things in their life than something bad, it is also natural that people do want to talk about something that is going wrong. But then, facebook is, of course, not a place to talk to your near-and-dear ones. You would rather directly talk with them than through facebook which is more like announcing to the world. So, what does facebook represent? I think it's a virtual place just to say "hi" and "hello" to your friends. And to share any interesting web content. That's it.