Simple example is when German intelligence service even before the start of the war was compiling details of Polish intelligentsia, cultural, social or political elites. So when they invaded Poland they knew where to look for Poles who could be a danger to the Reich in the newly acquired territories. And they executed them. One by one to destroy national identity. Now if back then something like Facebook existed, their job would have been much easier. This is what scares me, because when in future politicians decide that people of my views need to be "removed", that would be very easy to do. Just run a query against social media database and send kill squads.