The problem with social networks is that they present the same context for all sharing, no matter what the content. Even with LiveJournal friends groups and Google+ circles, your heart wrenching post about inner demons might show up between two lolcats and people think you're a let down when they want a laugh. Or your flippant lolcat might show up between an article about child abuse and a video of an earthquake and you come across as an uncaring douche.
In real-life sharing the people in the audience aren't the only determinants of appropriate sharing. The context they've created is also key. The things we share with a couple friends at a rock concert–passion, dance, exhaustion–are different than what we share with the same people in a coffee shop–analysis, discussion, confusion‐even though they're both done in public.
I don think big social network sites are able to tackle this well. The goal of UI designers is to create a simple mental model for users interacting with the system and the goal of software engineers is the create a simple operational system for interpreting user actions. Neither goal is helped by a flourishing diversity of contextual social norms. I hope all the bulletin boards and topic-focused sites survive in an ecosystem dominated by the Twitbooks. Because humans do really well when they can use location and appearance as cues to social behavior.