I actually don't think these numbers are important for Udacity, or for people taking the courses. What is important is that people are afforded the opportunity.

I actually don't think these numbers are important for Udacity, or for people taking the courses. What is important is that people are afforded the opportunity. It isn't Udadity's responsibility to force people through a system. Rather, they set a bar (payment) for some resources and then allow anyone who wants to use their material to do so. This isn't a classical university/college setup, where success of the institution is marked by how many people graduate. I'm not quite sure how we could evaluate their system, though, as these kinds of large-scale course offerings with low barriers to entry are a newish phenomena.