They're niche relative to "where else are you going to ask them successfully?". If you go on Twitter, whilst it may have way more people, you're probably not going to reach the same demographics in as meaningful a way.

I would assume so, yes, but the point still stands: he needs to moderate it or people won't find value in it, thus he'll be out of a position.

What's the value in Wikipedia? The curated knowledge. You only have to go on the average Talk tab for an article on Wikipedia to realize how hard-won most content is, and no, the best outcome doesn't always happen, and plenty of genuine nonsense still makes its way into articles and stays there for years on end.