http://www.npr.org/blogs/thetwo-way/2014/06/28/326453204/facebook-scientists-alter-newsfeeds-find-emotions-are-affected-by-it?utm_source=twitter.com&utm_campaign=npr&utm_medium=social&utm_term=nprnews
Facebook Scientists Alter Newsfeeds, Find Emotions Are Affected By It

For one week back in 2012, Facebook scientists altered what appeared on the newsfeed of more than 600,000 users. One group got mostly positive items; the other got mostly negative items.
Scientists then monitored the posts of those people and found that they were more negative if they received the negative newsfeed and more positive if they received positive items.
As the New Scientist reports, the research means "emotional contagion" can happen online, not just face-to-face. The magazine adds:
The findings are published in the current edition of the Proceedings of the National Academy of Sciences.
If you're wondering: Yes, this kind of experiment is in line with Facebook's terms of use. The Verge reports: