I don't know if the "too busy" thing actually would be that hard to estimate. Perhaps the US Time Use Surveys could illuminate things a bit? I've always (half-jokingly?) thought that a pre-requisite to becoming a serious lesswrong participant is being in a relatively unstructured period of your life (ie, unemployed, college student, semi-retired, etc) or having a completely broken motivation system which keeps you in a perpetually unstructured life against your will (akrasia).

I imagine that most of our potential target audience (those who would enjoy and learn things of value from the sequences) are already enough above the current sanity waterline of greater society to be so ridiculously successful (by narrow, American-style standards of success) that they no longer devote any significant portion of their time to reading recreationally... kind of like how knowing a bit about biases can hurt you and make you even more biased... being a bit rational can skyrocket you to a high level of narrowly defined American-style "success" where you become a constantly-busy, middle-class wage-slave who zaps away all your free time in exchange for a mortgage and a car payment. Nice job buddy. Thanks for increasing the GDP epsilon%.

Then again, most people, no matter how much time they were given, aren't going to read online intellectual non-fiction as a hobby. It just would never occur to most people as a possible idea.