I don't care what anyone says, the first time they encountered "You're the 1 millionth visitor" they clicked on it unless they'd heard about it before.
For most of us netizens though, it's so long ago that we consider everyone gullible who clicks on it while denying to ourselves that our first time is just further back than it is for most.
But those ads did not really do much besides being annoying. I've never heard of anyone who went through those ads and continued - because, 99% of the time, it redirected to some shady site that absolutely didn't play into the millionth customer shebang.