The (dis)advantage of Facebook is that most people aren't anonymous there, making posting stuff like "when I read feat X it felt like the designer slept with my mother, why wasn't he fired yet???!!!" slightly less encouraging given that your own mother may read it and your actual real life friends might discover just what kind of jerk you are.
On the other hand, it makes it possible for your mum/husband/boss/kid to see that instead of being a serious adult person, you are a man/girl-child playing with toy soldiers in your basement. Which will likely discourage many people, given the fact that our hobby is full of insecure introverts who don't quite enjoy the inherent extrovert narcissism of social media.
At the same time, Paizo boards allow for PbP, have several features missing from Facebook (spoiler tags, dice rollers) and well, they're Paizo's property, so they can police it the way the see fit and have all those nice boilerplates such as "all things posted here belong to us" or "unsolicited material won't be read" that help you out if you're in this kind of business.
And on the top of it all, there are all those people who kind of wish computers and IT in general never evolved past glorified calculators, and getting them to interact with you using all those newfangled...
I agree with all of your points. I think it is great the Paizo forums exist, however I am quite sure Paizo would keep on trucking without them. I still however don't see this as a major issue for Wizards who to me seem to be on the rise. Good points about the adoption or otherwise of IT amongst tabletop gamers.