It's frightening that agents from hostile regimes can create accounts on social media and news sites and pose as Americans or Europeans, and effectively create a shift in popular opinion. Just imagine how many comments one person working full time can create on major news sites in just one day, with many different identities. It's dangerous because it's not uncommon to gauge popular opinion by looking at what people are saying online.
2) Before the Internet, you would've needed to infiltrate individual friend circles with a large number of agents to make a dent, and they would have to be skilled liars. Now you just need people who can write untrue things, and they don't need to talk to many people to spread their subversive message, because the Internet does it for them.
@thor Comment sections were clear mistakes long before we had foreign agents using them for political aims.