So you launch a new search engine, well a reskin of the Yahoo! search results and call it Wabbadabba. How do you market it and get people to use it? Easy, you pay them. OK you don’t actually pay everybody but you give people the chance to win prizes - just for using your search engine. Not only that, but you encourage those users to sign up their friends, and if they win a prize then so do you. In effect a 2 tier referral scheme, how very simple but very effective.
So how do Wabbadabba fund the prizes? Well as they show Yahoo! sponsored ads then they obviously get a cut of the advertisers CPC. So all they need do is allocate say 50% of their profits for prizes and away you go. As the site gets busier, and more people search/sign up then you can offer bigger and better prize...