Hi Dinny,
There are several drawbacks to moving a forum to Facebook. The most obvious, as O No mentioned is that it is harder to maintain privacy there -- and it is harder to hide behind a pseudonym, so this would stop many from joining. Second, it is more difficult to maintain a dialogue on Facebook. Facebook is designed more for short hit and run kind of dialogues -- more chit-chat.
I have been on Facebook for about a year and have a Notes section to post articles and writings; but even that is not conducive to dialogues.
So, overall, I would vote that moving a forum to Facebook is a death sentence for the forum.
God bless, have a wonderful, blessed day,
Bill