Without a thoughtful moderator, purposeful progress of a discussion may not occur. As with any classroom discussion participants may opt to not participate or only participate at a minimum level. Required participation could be part of the course assessment. The level of genuine thoughtful involvement depends on the intellectual investment your students are prepared to make. While online discussions hold the potential for high levels of intellectual rigor, the risk is always present that discussion contributions will reflect minimal effort and thought.
Group size appears to be a key factor to the success of an online discussion.
If the group is too small, the discussion may peter out due to lack of fresh input. If it is too large it becomes unwieldy. The sense of continuity and community is lost, and less vocal members will begin to withdraw. As moderator it will become increasingly difficult to hold the strands together. It seems that in most instances, ten to fifteen is the ideal number of a group discussion.