Moderation is hard, genuinely hard, every order of magnitude increase in community size is not linear to the moderation requirement: it is factorial.
Why? Because every single communication has the potential for abuse, and the number of interactions on a platform do not scale linearly with the increase of users.
This is why things like the "Eternal September" exist, a deluge of new users is basically impossible to moderate at scale.
I think Twitter, Facebook and co. have done a fairly decent job of the mess they made, but crucially they decided that a walled garden where everyone exists together was their business model.
I think this is fundamentally flawed. "Back in my day" (I know it may be glazed with nostalgia, but) smaller close knit forums were much better at moderating communities, because it was still humanly possible.
There did exist some communities which became tyrannical; but the benefit of small communities is that people just go wherever it's "nice enough", and if you don't like the moderation staff or how they moderate you can move on with your friends.
I think people don't want this to be true, people are so financially invested in the centralised model; but ultimately you force a single set of potentially tyrannical moderators and a single culture. -- and people aren't willing (or able) to pay for the correct level of moderation.
It's Sisyphean and totally self-inflicted.