I asked the same question not long ago. I'm a mod on several forums. I've seen Facebook groups take away a lot of posters. People are already on Fb so they stay there. But I find Fb a horrible way to try to conduct a conversation. It really wasn't built for it. Traffic is down on lots of forums, including this one. And it's easy to lose people once they visit a few times and don't see much going on.
We're hoping to change that. We're taking some steps here (new forum software, more promotion, etc.) to make this a great place to talk hockey. After all, the playoffs and the Cup are coming up. You'd think people would want to chat! But there are a couple of folks around here, at least.