Of course they know it exists - But the point is is that everything on the internet is trackable, so they don't have to make predictions and guesses like TV advertising. If someone goes to a website and views/clicks on an advert, the webmaster will get a direct cut of that view/click. So if you block that advert then that webmaster will miss out on a bit of revenue from that view/click. On TV, however, an ad company says to a TV station "OK, well, we predict that when the commercials come on 70% of your viewers will mute their TV set, so only 30% of them will actually be able to HEAR our ads, and the rest will only be able to see them. And we predict that 40% of them will walk away to do something else when the commercials come on. So based on how many people we think will stick around to see the ads, we'll offer you XXX dollars." So if you walk away from the TV set when the ads come on, it's hardly hitting the TV station, as the $$ the station gets is based on predictions.