Facebook finds out what you think to share, Google finds out what you're doing and generally the path you took to get there.
Google News has always uncannily followed by current job's tertiary areas, for example.
- Google are the experts/masters of knowing "what you want", expressed via your search intents
- Facebook are the experts/masters of knowing "who you are" (including demographics, as well as family/friends/connections, interests/hobbies/personality, etc.)
Google can guess at the same type of "who" information that Facebook owns (e.g. gender inference), and vice versa Facebook can guess at the type of info that Google owns (e.g. search/purchase intent), but both have distinct relative advantages over the other, which is why there is still competition in this space (the GP comment was questioning, why hasn't Google just "won", why is FB even in the picture, etc.).
The power is in combining that data, like one of the grandparent comments mentioned. They will know if you buy a SUV (since the dealer uploads the data as an offline conversion) and that you're browsing baby clothes on Walmart/Target. Now you're a "new or expecting parent".
Some of these interest categories are not that explicit but the lookalike audiences are. That's a whole other post :)