Why is your company not leveraging social media despite overwhelming evidence that it should?

Are companies not supposed to be microcosms of societies where they operate? If yes (and I hope it is an emphatic one), why are they (companies) left behind the social media growth curve that is driven by the same communities they are part of? Better still, are company executives not part of professional social networks such as LinkedIn? If yes (and in my experience, a good number of the executives are, judging by the growth of communities such as National CEO Network), why do they not think it is just as important for the companies they run to embrace social networks?