Watching the Zuckerberg testimony to the Senate Judiciary & Commerce Commitee on C-SPAN2.
This Zukerberg guy seems to think that we're dumb. He keeps saying that Facebook only knows about what you share in terms of likes, dislikes, and pictures.
Here's something that they do: They track who's pictures you click on, how many you look at, and how long you linger on a picture or on someone's page, they also track who you creep on from people not in your friends list. They can use this to harass you and blackmail you. They can use this to determine your sexual orientation, and determine who you find sexually appealing, without you sharing the information explicitly, among other things. They also build extensive profiles on your political viewpoints, and track and monitor all of your connections and affiliations.
None of it is private. Don't use this service. It's a PRISM service provider, and can be used to triangulate your position and activities in the real world, and preempt and stop you from doing activities. RUN. Don't use it.
If you have to use it for business, be careful, be aware that it is an intrusive surveillance platform. Nothing you do on it is private, it's all tracked and stored in a database. They track all of your clicks, and database all of it and use it to determine information about your preferences that you don't actually share. Using very sophisticated statistical data mining. They also share this data with many more 3rd parties than just the Government of the United Sates of America. Don't use it if you can avoid it.