BE CAREFUL you don't catch those Facebook blues. Feelings, like viruses, can spread through online social networks.
A face-to-face encounter with someone who is sad or cheerful can leave us feeling the same way. This emotional contagion has been shown to last anywhere from a few seconds to weeks.
A team of researchers, led by Adam Kramer at Facebook in Menlo Park, California, was curious to see if this phenomenon would occur online. To find out, they manipulated which posts showed up on the news feeds of more than 600,000 Facebook users. For one week, some users saw fewer posts with negative emotional words than usual, while others saw fewer posts with positive ones.
Digital emotions proved somewhat contagious, too. People were more likely to use positive words in Facebook posts if they had been exposed to fewer negative posts throughout the week, and vice versa. The effect was significant, though modest
Even online, emotions can be contagious - tech - 26 June 2014 - New Scientist