I remember fondly the irc chat room stuff too... and I can't think of any time since the band has done anything resembling a direct chat thing like that.. (feel free to point me to anything if I am wrong). So this represents something well out of the ordinary, and my guess is, to open it up to the widest possible number of people / fans, in this instance, Facebook has been chosen as the medium.
Something like this would work well on twitter too, and it would be great at some point in the not too distant future we could get a live chat / Q&A like this here for subscribers only. Thats something we are always asking for, and pointing to requests for, by subscribers.
I see this FB Q&A as a big positive connection thing.. although I am not a big Facebook user myself, and its not everyones social media of choice, we can't deny its by far the biggest of them. And U2 like to do 'big'... Apple release being a great recent example of that. (love it or hate it, you cannot ignore it).
I love the idea of Q&A. Fans want to feel close to the band... so I guess it's very nice.
Of course there are two sides of using social media: one is that it reaches many people - almost 7000 comments of Facebook right now, but on the other hand it must be hard to choose questions, there are so many and also non-questions - some people just praised the band or asked when they will play in their country. At last, which is their aim? To reach as many people they can? So I think it's right to use social media. However, I understand that paid subscribers think they deserve more benefits than non-paid subs. But I'm just looking at the opportunity that was offered to go to Grahan Norton Show... so few people! Did you publish it in the social media? I guess no... so I'm thinking that maybe it might be because fans are not aware of it. That's the good and the bad side of social media. You need to see when worths to use it depending of your goal.