Facebook did not keep user data secret, they used it as a lure to attract Farmvilles and 'Which Hogwarts House are YOU' surveys to make sure people spent more time on the platform, because having a billion people look at the website every day is how facebook makes money. Using the data they have to make sure ads are efficiently targeted is a par for the course -- but it does no harm to facebook if someone else has all my user data, if I spend all my time on facebook, facebook is the only one who makes money selling ads to me.