If there is one consistent theme in both online and offline advertising, it's that ads work dramatically better when consumers have purchasing intent. Google makes the vast majority of their revenues when people search for something to buy or hire. They don't have to stoke demand – they simply harvest it. When people use Facebook, they are generally socializing with friends.
Chris Dixon wrote something I was formulating in the draft stage, and it's better. Search engines benefit from users' intent. Just go read his post.