Earlier quoted context omitted.
You can target Facebook ads with surgical precision to manipulate people exactly the way you want. This is exactly what Cambridge Analytica did. You cannot target people with such precision in traditional advertising. I don’t think it’s unfair to say social media advertising and data collection is the biggest risk to democracy that we have ever faced.
The targeted precision really is what's different. Your candidate can be N different single-issue candidates in a way that's just not possible without precise targeting. The "echo chambers" explanation, in particular, seems wrong. Echo chambers always existed.
Before near real-time national news media coverage and the ubiquity of portable recording devices and online media to distribute the recordings, that was normally the case, but the segmentation was geographic and by addressing selected audiences in closed events. Many political gaffes and scandals of the last couple of decades have been candidates getting caught with unexpected exposure of addresses to either specific geographical markets or closed-group events that were less palatable to general audiences. It won't be too long before political groups maintain social media personas with a variety of constructed backgrounds to capture and expose to wider audiences to which the message would be repugnant those ads that the other side targets narrowly.