I see some old school mentality going on. It's like those that asked why anyone would bother with DVD over VHS. Or cd over cassette.
The fact is Facebook is much easier to promote on and handle. It's also free. You don't have the overhead of a website such as forum subscriptions, server costs, and it takes less time to maintain it, etc etc.
Facebook may not be for everyone, but if you ignore its relevance then long term you could face consequences of lack of adaption. Especially as it continues to grow and evolve. Because obviously it's starting to head into Reddit and forum territory. It's like ignoring that robber just because they are on the border of your fence but haven't yet fully crossed over.
If forum software doesn't find a way to keep innovating or adapting that will be troublesome. And if it integrates Facebook too much then many will wonder why not just go with Facebook.
It's a balancing act. But ignoring it's importance and relevance is dangerous.