I think it's important to reach out. You need to make your presence well known in an online classroom and you can do this by providing customized feedback in the gradebook, in-depth discussion replies to students that further the discussion, and even encouraging emails (without emailing any sensitive or confidential data). Sometimes my students will get 100% on a quiz, and I'll shoot a quick email - "Great job on the Week 3 quiz - your efforts were noticed!!" Something as simply as that has yielded positive comments from students. Of course, within the quiz itself or gradebook, there should be more detailed feedback about ways to improve and areas that were done well. In addition to using the students name and specifically address points earned/deducted (instead of generic responses), I also try to use the 'sandwich' approach in the feedback. The 'sandwich' approach is to praise something done well, then critique an area (if needed), and end with more praise...