There are ways and ways of paying for ads. If they take out a straight ad and pay a flat rate, they will judge the site by average numbers shown to visit, which appears in site statistics. If they do it by paying "per click" it is by how many people click the ad to go to their site and visit (and it is only a fraction of a penny per click) It isn't by how many see the ad on here, if they don't click on it.
The competitions will be mentioned on the websites where competition junkies go for their fix, so will attract people to come here and join so as to enter.