If you use Facebook, you may have been an unwitting subject in a massive psychological research study.
As detailed at The A.V. Club, Facebook manipulated over 600,000 users' feeds to see how feeding them bad or good posts would affect them. They did this by tuning the algorithm they use to determine which of your friends' posts you get to see, selecting for negative or positive words, then observing the user's own posts. Turns out it works! When they fed you negative information, you were infected with what the authors termed "emotional contagion" and became more depressed and angry. On the plus side, I suppose Facebook might have cheered some of their experimental subjects up as well, so let's hope that was you.
Did you sign up for that when you signed Facebook's End User Licensing Agreement, which permits them to use your data for "troubleshooting, data analysis, testing, research and service improvement"? If you don't think manipulating your emotional state and observing the results qualifies, then you probably didn't give "informed consent." Psychological research generally demands informed consent from its subjects, because messing with people's emotions without their permission or awareness isn't very ethical.
But that doesn't apply to you, because you signed a EULA, which you probably didn't read anyway. You weren't the subject of psychological research whose mental state was manipulated without your consent, you were an end user who took part in some innocent data analysis! Here, watch these videos of cats falling off tables and dogs being reunited with soldiers. Feel better?