Many forums have lost members to facebook. The bigger forums seem to be doing fine, as they can take a hit and keep going, plus they have made themselves part of the communities they represent, by effective modding (read: not letting the inmates run the asylum). That way it's hard to do things without them.
But the smaller forums, on which people often know each other or are at least online friends as well, tend to see traffic move to facebook, because their members talk amongst themselves there as well. What's the use in having the same conversation in two places after all?