alt Hacker NewsI think that the argument is that social media amplified the effect. It obviously didn't create social pressure, but pressure was limited, and people were just free to be more diverse--or, to put it in the opposite way, there was less pressure to conform.
When something bad happens (like, say, a murder), people want to talk about it. But now, the conversations happen in view of the public--the _whole_ public. So people don't want to be on the wrong side of the conversation. In private, among friends they understand and trust, they might express doubt or complex thoughts. But out in the general public, where their opinion could be boiled down to a soundbite, they don't dare lean on nuance. So, they play it safe.
Go onto Reddit, and under some controversial story post a nuanced take. Chances are you'll get downvoted to oblivion. And that's fine--it's pseudonymous. But if it were attached to your real name, and you happened to be wrong (or some part of your statement could be taken out of context and made to look wrong), then it could always be dredged up and used against you. OTOH, there's basically no scenario where it would be dredged up to help you.
So the result is, all nuance is drained out of the conversation. We end up with junk food for public discourse: no fiber, no nutrition, just feel-good sugar: "fuck that guy, he should be locked up for life!" And seeing everybody carry that line unanimously makes anybody who doesn't look immediately suspicious.
I don't find this particularly convincing as a fundamental change in how social pressure in communities works. I grew up in a suburb (around 25,000 people), and social pressure didn't need social media to cause real-world consequences; a priest at a church got reassigned and later quit being a priest altogether because of a rumor that he had an affair with a married woman, which in reality never happened and was a rumor started by the husband with no actual evidence. No one discussed any of this publicly at any point, and yet everyone knew that the priest was well-liked at first, and then everyone knew that he was reviled right after he got reassigned, and then everyone knew that everyone felt bad for him when the truth finally came out.
I'm not saying that social media can't bleed into real life, but I think the effects it has on how communities choose to ostracize people is completely exaggerated here. Your high-level theory for how it could result in a huge societal shift might be plausible, but it would also be plausible to argue that it causes people to move on much more quickly from events like this because it's so much easier for a new one to pop up for everyone to focus on, which leads to people no longer caring about the previous one faster. All of this is is just armchair philosophizing though, without any real evidence.
I think there are very real problems caused by excessive social media that are documented with compelling evidence, but I have trouble believing this is one of them.