"When positive expressions were reduced, people produced fewer positive posts and more negative posts; when negative expressions were reduced, the opposite pattern occurred. These results indicate that emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks."

Facebook were able to carry out the experiment because all users have to tick a box agreeing to their terms and conditions. These include "internal operations, including troubleshooting, data analysis, testing, research and service improvement."...REST OF IT HERE