I have to call shenanigans on this. My social circle regularly has huge debates via Facebook links and wall posts; off the top of my head, I can recall discussions about birth control, existentialism, web frameworks and the impact of video games on our culture. Social networks are tools; there is no prescribed way to use them beyond the actual codebase they run on. Yes... Facebook's design <i>affords</i> phatic throwaway talk more easily than it does discussion, but that doesn't mean you can't use it any other way.
Social networking isn't about a conversation, that was 2007's cute mantra. Social networking has devolved into a self-promotion platform whose only existence is to advertise. The conversation ended years ago. Don't worry, though, the bubble is popping. Soon we'll have some new ridiculous hype so that silly people can make lots of money being doofuses online by calling themselves "web 3.0" evangelists or whatever.