Facebook revealed that in 2012 it allowed researchers to manipulate newsfeeds of hundreds of thousands of users to influence their emotions and track their subsequent behavior. All of this was done without users' permission, and the findings eventually appeared in the study "Experimental evidence of massive-scale emotional contagion through social networks."
Over the course of a week in January, 2012, a team of scientists-including a Facebook data scientist who posted defense of the experiment on Facebook-hid elements of almost 700,000 users' profiles that contained emotional words like "love" or "hurt" according to The Guardian. The researchers then studied how that affected what users "Liked" and shared on the social network. Eventually, the researchers concluded that users' moods and emotions were reinforced by "social contagion."
Predictably, there has been a backlash, especially among the research community, which says the researchers have an obligation to get informed consent from participants. According to a blog post from James Grimmelman, a law professor at the University of Maryland, "Whatever you think of Facebook's ordinary marketing-driven A/B testing is one thing: what you think of it when it hops the fence into Common Rule-regulated HSR is quite another. Facebook has chosen to go walking in a legal and ethical minefield; we should feel little sympathy when it occasionally blows up."