I don't really know as I'm not on Facebook but from what I heard, everything you put on Facebook belongs to them. This includes messages, likes, videos, music, photos. If you could put your dog in their servers they would be the owners of a dog.
And they can use the photos you put there as a marketing tool to your friends.
They can also recognize you with a very high success rate simply by your posture. Their algorithms can detect the style of clothes you wear and use it to categorize your style. By knowing your friends, they can make a nice profile of you. And through their Instagram branch, they can detect the mood you're in with a high degree of precision simply by looking at your picture's metadata.
It's all fascinating but scary stuff when you think about it.
