Anyone remember AOL chat rooms? You'd jump into a room with a topic (#teenpoolparty13) and start chatting with strangers. In general it would be okay, because it was like a bunch of people huddled together trying to have their conversations in one tiny bedroom. "New" social media killed the free-form group conversation, instead creating splinter conversations that were so highly focused that it actually generated more conflict.
Even in-person, where we have a thousand interpersonal heuristics and social queues to control social interaction, it can still be a mess. Social interaction is best in small groups connected by a common theme and protected by a blanket of familiarity. But with a random group of strangers, no social (or physical) ramifications, with splintered, hyper-focused conversations, it can only either A) devolve into rancor, or B) become an echo chamber. Bringing together lots of people you barely know or don't know at all to talk about random things just isn't a good model for human interaction.
Eternal September is just one of many of the examples of it devolving. With a small consistent group you can establish a consistent culture and psychological safety. Grow the group and the newbies destroy it. Healthy social interaction relies heavily on trust and cooperation.