Amazon does much the same thing with its recommended items, and it's cute. I'm sure it drives sales, and by extension would be (is? Not sure if Amazon sells ads in that context) worth advertising to. But realistically it's got to be a small part of Amazon's revenue. And Amazon's revenue dwarfs Facebooks.
Honestly my intuition says that there isn't that much money in that kind of "buy this, you don't even know you need it" advertising. Consumers generally know what they want already.
"Buy this, you don't even know you need it" is basically the foundation of American consumerism. It's the concept behind dandruff shampoos, pet rocks, and designer clothing.
BTW, Amazon's revenue dwarfs Facebook's but Facebook made more profit last year than Amazon ($1 billion vs. $900 million).
Honestly my intuition says that there isn't that much money in that kind of "buy this, you don't even know you need it" advertising. Consumers generally know what they want already.