Facebook once came under heavy fire for revealing that it had run a week-long experiment on its network that intentionally manipulated the emotions of users without telling them. The purpose of the experiment was to find out how mood transferred through social groups - would seeing a sad status make people feel sad? What about happy updates?To test this, Facebook experimented with nearly 700,000user accounts, by posting either more negative or more positive updates on their news feeds than normal. They then tracked what all the users posted after they viewed the news feeds.