I really like the suggestions on how to handle a quiet learner. In particular, I think sending an email to the quiet learner that refers either to their biography or something else you know about them that would be valuable to the discussion makes the learner feel valued. The value thus imparted by the instructor with the email can increase the confidence of the quiet learner. Indeed, the quiet learner may post something of exceptional value as a result, thus helping out the entire class.
