Teenagers have been bullying each other for literal millennia. Kids got bullied on IRC and chatrooms; they get bullied today on Roblox, on SMS group chats. None of these have engagement algorithms.
Again, the corporations sure aren't helping, but we need to confront the fact that a large chunk of this is human nature. I'd like a better Internet landscape but switching from Facebook to Mastodon or whatever, while a good step, isn't going to cure all ills. A lot of people in these threads act like every Internet toxicity symptom ever arose because of evil corporations.