Facebook ran a mood experiment on 689,003 users without telling them

For one week in January 2012, a Facebook study quietly reduced either positive or negative posts in the News Feeds of 689,003 users to see whether emotional states would spread. Published in PNAS on June 17, 2014, the experiment found small contagion effects, but it became far better known for its ethics: the users had not given informed consent to be experimented on. PNAS later issued an Editorial Expression of Concern noting that the research may not have met standard principles of informed consent and the opportunity to opt out, while stopping short of retracting the paper.