Because there is no reason to believe that the % of men who have true ED matches up with the amounts of Viagra sold; because I've known many young men who talk about getting drunk or high and taking it, and because their companions agreed or in the least were not surprised; because the market for it online seems to be mostly young, healthy men who want to have greater stamina, not who have anything dysfunctional to begin with...
You don't see the tremendous amount of selection bias in your sample?
Very quickly: people who need viagra don't like to talk about it, unlike drunken frat boys. The online market is targeting men who can't get it from their doctor because they don't have ED. That doesn't mean the vast majority of viagra users aren't suffering from ED, it just means that online ads are targeting a niche market.
