Facebook is under investigation for reason: broke data protection laws when conducted a study named "Emotion Study" on users without their consent.
FACEBOOK FACES UK INQUIRY OVER EMOTIONAL STUDY
Facebook conducted experiment named "Emotion Study" in January 2012 proposal by Social Network in cooperation with Cornell University and University of California. As reported by study: "The experiment manipulated the extent to which people were exposed to emotional expressions in their News Feed". In poor words this experiment shows how emotional states can be transferred to others via emotional contagion.
The Information Commissioner's Officer (ICO) said to be looking into this study that "manipulated" almost 700.000 Facebook users. The ICO said that is too early to speak of laws violation for Facebook but question is over the study.