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To obtain the type of responses you desire, you should set minimun expecations on your students. You should tell your students the miniumn number of words that is expected in their main post as well as how many follow up post. You should give them examples of what is expected. A follow up post should consist of more than "that is cool" or "I agree".
You could create a rubric and post it on here for students to reference.
                                
                                    
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