Brian Leiter, who only opens comments on his blog occasionally and moderates them well, posted a funny list of descriptive laws that mostly speak to this point. Public Facebook pages, Youtube comments, and so on are generally unmoderated or close enough to unmoderated that they turn into complete trash. At most people spamming advertisements, obscenities, and other obvious trash are removed. Forums that also enforce things like staying on topic and actually making a point (if the thread is about arguing) or otherwise contributing a net positive to the conversation are generally better. (Which is why when I have moderation powers over a thread, I’ll cut off post chains that look like they’re going nowhere good, even if to the dismay of the poster.)
