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"2 million unique students are viewing our videos every month by choice"

Page hits isn't a very good metric if you're trying to get feedback about what viewers actually thought of the videos. A lot of people have heard of Khan Academy through the news or may land there via a search engine. How many of them watch the video all the way through? How many stick around and watch parts II, III, etc.? I could be totally wrong about this (please correct me if that is the case), but my suspicion is that YouTube's stats probably aren't sufficiently granular to distinguish between a page visit in which the viewer watches the video in its entirety (suggesting that the video was perceived as worthwhile) and a visit in which the viewer watches for a little while and then bails (indicating that it was not perceived as a valuable experience). User comments are subject to a great deal of self-selection bias. Viewers who thought the video was the best thing since sliced bread are way more likely to take the time to login (if already registered) or register and login (if not already a registered YouTuber) than those who thought it was a waste of time.

There's also a more fundamentally important question to consider: Did the viewer actually learn the material or were they simply duped into a false belief in their own competency? HN regularly features posts about interviewing techniques aimed at sorting out real programmers who can write code proficiently from people who simply possess a false belief that they are programmers. Sound pedagogical methods require some means of verifying that the student has, in fact, built a valid mental model of the subject matter and, of particular importance in the sciences, is capable of applying their knowledge to reason about novel situations. How do you know that your students actually understand the material? Do you have any way of distinguishing between students who have had a previous formal learning experience with the subject matter and are simply looking for a refresher course, students who are concurrently receiving other instruction (the teacher is using Khan Academy as a supplement), and students who have had no prior exposure to the material?

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Wow. Here comes the bury brigade to crush my dissenting opinion without bothering to address anything that I said... How dare I criticize the sacred cow? How dare I challenge conventional wisdom? Conform, conform, conform. Everybody loves Khan Academy.