The number of organizations that have given up other means of organizing for FB groups is, well, far too many. With Yahoo Groups gone and Google Groups having the Sword of Damocles hanging over it (not to mention the issues average folks have with using email discussion lists and their effective deprecation by anti-spam measures), FB groups looks like an easy option to people, and suddenly to participate in an activity you need a FB account, or a very communicative friend with one.