I'm torn. I already spend far too much time there. In my experience, this could potentially lead to more dynamic conversations than on other platforms, in part because the Facebook group interface is something between a webforum and a webchat. It would also be easier to share audio, video, and photos than it is here - it is an inherently richer medium than text alone. But I find that social media, for whatever reason, can bring out the worst in otherwise reasonable people. Because of the way Facebook works - people can delete their own posts and comments, which consequently deletes any replies - there is a potential for people acting maliciously. This *will* require active moderation to combat.
Aside from that, the conversations would be hosted on a commercial platform upon whose mercy we would depend, and one that is not, to my knowledge, easily archivable: even if the group is public, I don't think Facebook allows robots to index or backup its contents. (This strikes me as particularly ironic...) We can hack together something that could do it, I guess, but my guess is we'd need to constantly maintain it to account for changes in Facebook. The search feature is also limited - it searches posts but not comments on posts, as far as I can tell.
That said, I think there could be room for both a Facebook group - perhaps for more casual conversations? Or for link-sharing, as on the AMIA and IASA groups? - as well as the traditional listserv. It can potentially enlarge and enliven the community. Much is posted on Facebook by ARSC members already that would be of interest to other members, on their own walls, and a group could serve as a clearinghouse for such content. On balance, I think it makes sense to at least try it, but we have to go into it with cool heads and clear eyes.