Try blog discussion leaders. I do a lot of group work, so one approach has been to have each group responsible for posting to the class blog at a different time. Perhaps in response to an assigned reading, or a reading of their choosing. If class is on Wednesday, I would have each group member post a blog by noon on Tuesday. Then everyone, including those that posted originally, is repsonsible for posting so many comments by class time. This begins the conversation outside of class. As the teacher, I respond with just a few comments. Some directly to the original weblog. Some in response to a comment.
This is a similar model to what I have done with discussion boards, but I think blogs may facilitate this more because someone can scan the different posts, rather than just forum topics, and choose which to respond to.