I don't see how this could work without a large base community. Reddit has tens of millions of users, which makes the AMA concept work (win/win for both sides). However, Reddit AMAs aren't that mainstream though, and there could be a way to extend this concept into the reach of Facebook's billion+ users. So, I think the only way to make this work would be tight integration into Facebook, and possibly a mobile app. Monetization strategy could be to make the AMA hosts pay depending on the size of the reached audience.