Although your method of address the entire class is certainly the most efficient from an instructor's viewpoint, it may not fully address the individual needs and diversity of your students. While there is usually not enough time to address each post (nor should anyone usually), inserting a well-placed comment to emphasize a point or to redirect the conversation is extremely helpful to reinforcing your credibility with the group. Certainly try not to "play favorites," and address a variety of students as warranted.
