Every day, millions of young people swap selfies on Snapchat, update their status on Facebook, and swipe right on love interests on Tinder.  For millennials, social media is essential for a 21st Century social life — but for 62-year-old political scientist Eliot A. Cohen, it apparently means the end of the human civilization as we know it. For malevolent actors ranging from the government of Vladimir Putin to the recruiters of the Islamic State, social media is a weapon which can undermine American democracy and even topple entire governments.