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. You can put billboards all over a park, and maybe sometimes you’ll happen to convert people from non-purchasing to purchasing intents. But you end up with a cluttered park, and not very effective advertising.
Really great (short) post that highlights a key problem for Facebook going forward.