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.