It doesn't really matter that it is specifically Facebook but that cocoon & comment type social-networking system is not what a "forum" should be, or at least I don't think many of us started forums with that mentality nor will IPB win any battle with that type of service.
I'm all for cherry picking good ideas and adding them if they work with the forum concept or lead to enhancing it. Login with your Facebook / twitter info? Sure why not , just makes it easier to login. Allow easier integration with the social networks to "advertize" your forum? Sure why not?
But morphing the whole thing into a cocoon type system is , while perhaps "popular" missing the forum discussion concept.
First half of this quote covers it although I wouldn't quite go as far as to say Facebook-type discussions and forum-type discussions are mutually exclusive (they do share some similarities) but they are very different and "diluting" the forum concept by slowly morphing it into a poor-excuse for Facebook is not the way to go.
To restate: taking some "good features" that lend to easier promotion / facilitation of forum-style open discussions from facebook and other such places? Absolutely. Try to turn forums INTO such entities is the bad move.