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.