[dredmorbius]

I don't know that you've hit on the whole dynamic, but you've hit a lot of it: scale.

In 30+ years of online engagement, I've seen again and again and again numerous transition points for communities as they've grown and hit various sets of problems. These seem to pop up fairly regularly at specific points. The small group (a handfull, maybe two), then the social circle of 20-50. Most online groups seem to fist start hitting behavioural issues that surpass a single admin/moderator after a few thousand or so active members, maybe 100k on the outside.

Facebook, in one view, was successful in surpassing these limits, but in doing so it's discovered others.

Scale matters.

As does speed, cycle time, UI/UX dynamics, incentives, (media system) memory, and so much more.

And it's not just anonymity. Not by a long shot.