But there's more to it than that. Lectures suck as a means of learning about red-black trees. Unless the lecturer is literally world-class, I can better spend my time by watching the world-class version of the lecture on iTunes U. Or by reading the best book on the subject -- books move faster than lectures, except when you need to slow down and think, in which case they obligingly slow down for you.

Of course, while lectures are a fairly low-efficiency way to learn many subjects, it is necessary to give them at least a little attention in order to learn about what will be on the specific test that the professor will give you at the end of the term. And, of course, some lecturers are world-class. If you find one, put Facebook away, for god's sake!