Facebook doesn't just have the social graph of who your friends are and what they search for.

They also have an increasingly insightful record of your personality. Probably moreso than Google, since Facebook gets more of your "casual" life than Google.

Google is in a good position to record and analyze the intents you express, but Facebook likely has a better record of who you are outside of those expressed intents.

Turning that into a useful product is difficult, but I'd be very surprised if they never work in that direction.