Everyone is susceptible to propaganda, including you and me. Democracy is based on information, so if you mess with that information, you can most definitely have a noticeable effect. The propagandists exploit the asymmetry of resources and time between them and you. They can lie easily, they have all the time and money to craft careful lies. You don't have all the time and money and domain knowledge to figure out each and every one of their carefully crafted lies. Not to mention other more sinister tactics like tracking you, finding out your biases and using them against you with targeted propaganda campaigns.