Social media turned the whole world into a high school cafeteria.
But what you embrace as historical and perennial in shunning, shame, and ostracism is in like historical and perennial company with compassion, forgiveness, mercy, etc.
But for many of us in the US, in the GenX and Millenial generations that we grew up in, we experienced the worst of the shunning culture in high school, among small town cliques, and in the midst of certain church groups; and then social media came, and we witnessed a clear turn where the dreaded "Mean Girl" dynamics that we hoped to have escaped in a liberalizing adult world of funky tattooed weirdos, long haired UNIX nerds, rainbow-flagging Unitarians somehow became the dominant voice of social media culture and it was somehow being championed loudest of all by some of those same colorful weirdos!
Maybe you're happy with that turn, but you can't really take away the disappointing and disorienting transition that many of us experienced as it took hold.
The colorful weirdos were never better people in the first place, so I don't know why you're surprised. People who don't presently have the power to exploit others tell themselves that they don't, or wouldn't anyway, because they're better people. This is self-flattery. Give them power and they'll promptly demonstrate their corruption.
Nietzsche wrote quite a bit about this dynamic.
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'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.