Briefly: more often than not, people do not have a clear idea what they need or how to ask for it.
So my response to the recent resurgence of online community Q&A services, like Facebook is trying, is that they are probably doomed to failure. Not to say there's no place for such services. I think, especially within niche communities, they serve an invaluable source of support and information. But it takes training and experience to be able to guide most general information seekers to uncover what they really need. In my opinion, this cannot be effectively accomplished, beyond rudimentary questions (which can mostly be answered by simply "Googling it" anyways), by anonymous voluntary answerer's or by reams of FAQs.
These are skills that trained librarians and other specialty service personnel do best through what we in the library world call the "reference interview." It is a give and take; a negotiation; and a teachable moment. So I'll be watching Facebook's efforts in this area. I don't think the service (or others like it) are any sort of threat to libraries, especially if we continue to get the word out about our value and skills. Granted the library profession still has lots of room for improvement in that department.