Sarcasm really doesn't help online communication. I think they're referring to a phenomenon called "dogpiling" where a twitterer says something about a person they don't like, and their followers have a field day trashing the person. The phenomenon isn't new, but the magnitude is.

Come to think of it, maybe just the speed and availability. Mass letter writing campaigns are totally a thing, but they require significant efforts to coordinate

The push (vs pull) / broadcasting nature seems different to me.

Previously, more interaction was personal and ephemeral. If I said something to someone on the street, maybe the 5 closest people heard me.

In a small town, word would probably get around in ~2 weeks of social interactions, if it was scandalous enough.

Now, on the majority of social media platforms, if you reply to me then all of my friends + all of your friends immediately see the reply. Both the broadcasting and immediacy seem relatively novel.