-More than 680,000 Facebook users were part of a psychology experiment in January 2012 without their knowledge.
In order to investigate whether the emotions of other social media users could lead "people to experience the same emotions without their awareness," researchers manipulated 689,003 users' News Feeds to show statuses that were especially positive or negative.
The results were published in the Proceedings of the National Academy of Sciences in an article called "Experimental evidence of massive-scale emotional contagion through social networks."
"We provide experimental evidence that emotional contagion occurs without direct interaction between people (exposure to a friend expressing an emotion is sufficient), and in the complete absence of nonverbal cues," according to the authors.
It turned out that users exposed to negative content were a fraction of a percent more likely to produce negative content, and those shown more positive content were more likely to produce positive content -- though a fraction of a percent on the scale of millions of users is not so small.