Typically, use Web Forum and chatroom discussions to supplement in class conversations or for discussions that can help the class but are not crucial enough to spend class time on. As I said above, make sure you have clear ideas about what you want a discussion to accomplish and what expectations you'll have of students. One effective use of the Forum and chatrooms could ask students to "pre-discuss" an article or assigned reading, thus allowing the teacher to get a sense of where students are at (and potentially how many students have read the article) prior to the in-class discussion. Once a discussion is started, make sure to monitor posts to not only record who is participating, but also so you can perhaps guide a discussion a bit more, prevent inappropriate online comments (flaming), keep the discussion focused, or move the discussion in the direction you want it to go. In short, you should probably still serve as a moderator of sorts, though the control you exercise can vary based on your goals for the discussion. No matter what role you choose, don't just leave the discussion to happen on its own.