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 affords phatic throwaway talk more easily than it does discussion, but that doesn't mean you can't use it any other way.