There are unique advantages to online classroom discussions - the pace of thinking is better for thinking deeply, and the quality of the discussion can potentially be much richer. Here's one paper we happened to come across.
In our experience, teachers and students are just figuring out what they can do in this medium, but from our viewpoint, it looks like a powerful medium for introducing reflective thinking even at early ages. Our personal experiences with online classes have included CTD Northwestern, Stanford EPGY, Great Books Academy, and Islas. The ones that mix asynchronous discussions with live 'chats' seem to be the best.
Critical Thinking Through Online Discussions>