I'm all for it. I think it could be cool to have a facebook group in addition to the site.
I'm just kind of scared that if it's on Facebook, the site will die from inactivity. My friend and I tried starting a forum site for a gaming FB group that already existed and the site backfired with inactivity. But it could be very different for this forum, being that we already have active members and being that it's an atheist-based forum.
I say it's a great idea though especially if take the right approach. I'm in.
Would the group be open? Upon request? Invitation only?
Also, when posting... is a post made to the group visible to that person's friends and people following them? Just some things to think about.