I wouldn't say facebook is killing forums, It could make it harder and push you to bring more of a diversity instead of the simple "Forum". Facebook has major issues when it comes to spamming etc etc and turning away a lot of people...
If you can't get rid of the competition, join it...

and you will have to go back to the drawing board.
Twitter is another one. Twitter and Facebook but I haven't lost that many users just that they do not remain on the forum as long.
I think forums needs to be a bit more light weight and twitter showed me that no matter how many features you add, people are still looking for the quick and faster issue.