The use of social networks by startups, terrorists, governments (of all kinds), intelligence agencies, (and remember spammers?) is extrememly effective because the systems were built for viral marketing campaigns. The Brand is You. But also all these other things.
These new social systems are more instantaneous and powerful than our existing systems, and therefore most of our existing systems, if not protected, will fall.
(Apologies as I have no idea how to capitalize coup d’état correctly.)