IMO, forums can easily co-exist with Facebook. Obviously, you have to adapt to the changing environment.

Let say you received a ton of page views from people having casual conversations on your forum. Well there are much better mediums for people to have casual conversations. In most cases, you can't expect to offer someone an inferior service and gain traction. Rest assured that quality sites see the reverse effect; they gain traffic from FaceBook.

Think, what does your site offer people? You should have several advantages over FaceBook; Quality content, articles, in-depth analysis, custom coded features, etc. There are several things that you can offer that are dedicated to your specific niche that FaceBook can't.