The ad rating interface shows people ads according to an algorithm. Our software will ensure that every ad will be viewed enough times for a fair rating, and that no user views the same ad twice. After each ad has been rated a number of times, the algorithm will assign more popular ads to voters more frequently than less popular ones.
Ads are not indexed to prevent people from getting all of their friends to vote for their ad -- which wouldn't be a very good way of finding the best ads.
With only 15 finalists, this process allows us to know with greater certainty how the better ads relate to one another, while also giving ads at the bottom the chance to rise to the top if the first people who rated the ad were wrong. Any given user has an opportunity to rate many ads.
There are also other mechanisms that, for obvious reasons, we're not going to disclose here that will prevent people from gaming the results of the contest. And we'll be doing post-rating analysis to make sure there was no funny business.