[tragic]

That certainly could be it - it's literally the only graph I could pull up at short notice. There are stats around but like browser usage etc they tend to come from this website or that. Based on some other numbers[0], trends seem to be more or less in line with what you'd expect: more blockers installed for visitors to tech-related sites; more installed on Firefox than IE, and Linux than Windows; etc. In that link, you're looking at nearly 10% of impressions blocked as of 3 years ago (which is obviously not the same thing as unique visitors, but I would be surprised if that was not higher than a few years before that).

I'm merely stating a hypothesis here, really - I have in mind how it was that people installed things like the google toolbar to block popups, and then browsers blocked popups by default, and then there were no more popups (OK, not quite, but still). The more irritating something is, the more likely a user will take steps to do something about it.

Time will tell.

[0] http://www.quora.com/What-is-the-percentage-of-Internet-user... - sorry for the hated Quora ink