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.
Google seems to have forgotten that their strength doesn’t necessarily lie in the quantity of data that they have about users, but rather the right data.
Google can present me with an ad when I’m actively searching for something. Facebook tries to shove an ad in my face when I’m trying to do something different.