Gillian, thanks for your comments. I think we need to determine what learning is. Yes, an old question, but if we don't know what we are looking for we can't measure it. To me learning is not about the number of posts but what students do with it. I think we need to start with the end in mind - assessments - and rich assessments such as reflections, creations, arguments, etc. Going backwards, we can research the tools used by students to arrive at their understanding and work. If situated in the cloud and accessible, we can review the types of interactions, materials explored, and tools used.
It seems to me most people in here are saying the same thing. Student action in a LMS is only a part of their learning. With a diverse and wide spread networked world, we need to reconfigure how we capture and analyze data about student learning.