Its an insult the way they're going about this. Pretending that the majority of their users would prefer to use the cancer that is FB/Twitter, which even if you wanted to, is in no way even close to being a substitute to having a board for every single movie/tv/actor/cast ever.
The cost to run these boards is less than a rounding error to Amazon, they waste more money on their many projects that never go anywhere.
The real reason is that the people who actually use the boards are dedicated, you can't sell these people's profiles and 'monetize' them, you can't exploit them, and you can't sell them other junk.
I don't even care about moderation - any big forum will have trolls, let people self moderate.