The difference is that until recently, you saw this stuff on Fox, and other stuff elsewhere. Sure, some people (a lot of people!) only watched Fox and swallowed it all, but a lot of people didn't. They knew that Fox was Fox, etc. Nowadays, in a pile of YouTube comments, or Twitter replies, or Facebook posts, you cannot tell who or what is behind it. You know when a story is broadcast on Fox that it's Fox. When it's a comment, is it a real person? A bot? A troll-farm worker? If one of the latter, where is the money coming from? There's no way of knowing. Add to this group polarization[1] and other known psychological factors, and it's easy to flood out a particular online space with a particular narrative, with no visible accountability.
1: https://en.wikipedia.org/wiki/Group_polarization