Set up course discussion so that it's a learning experience, and one that students create together.
First, review some guidelines:
Engaging Students in Discussion Online
and get ideas for assignments from this Penn State guide, kept alive by Prof. Bill Rozaitis at University of Minnesota.
Use D2L discussion, or set up a Google Group or where students can discuss topics. They can "subscribe" to the group (via RSS feed), and get/send updates as email without having to check the site regularly.