Facebook's secret mood study was published in 2014, detailing its emotional contagion experiment conducted on 680,000+ users without their knowledge or informed consent. Facebook wanted to see if it could control the emotional frame of mind of users, by manipulating news feeds.
There was a wave of negative feedback from media, users, and academics. Facebook claimed that all its users had agreed to be research test subjects by signing up to use Facebook. Yet, Facebook did not have a research clause in its terms of service until four months after the experiment was concluded. Facebook apologized for "the way the study was communicated."