As of now, the best way to discuss recommender-system topics is probably Twitter and the Hash Tag #recsys. To the best of our knowledge there are no other (active) forums, mailing lists etc. for the recommender-system community. The only recommender-system specific forum/group we are aware of is the LinkedIn “Recommender Systems and Software Engineering Group” but it only has 75 members.
Contact us if you do know some other active forums etc.