Synchronous and asynchronous discussions are vitally important for online learning. Motivating students to participate in them is often difficult. I think the key is in the questions that the teacher designs, the consistency in the moderation of the forum and that the instructor considers evaluating the participation of the students with degrees. It has given me good results to create teams and appoint a boss for each one, so that they evaluate each other and themselves. It is intense work, but it usually works if you give it the right importance and the topics to be discussed are problem solving, critical thinking or controversial issues.
