Facebook's News Feed [the main list of status updates, messages, and photos you see when you open Facebook on your computer or phone] is not a perfect mirror of the world. But few users expect that Facebook would change their News Feed in order to manipulate their emotional state. In the report, the authors explained they had 'informed consent' from users because altering the information was consistent with Facebook's data use policy, which all users have to agree with to have an account. The experiment is almost certainly legal. In the company's current terms of service, Facebook users relinquish the use of their data for 'data analysis, testing, [and] research.' Is it ethical, though? I should point out that I do not have and have never had a Facebook account, mainly because their security provisions are pathetic, however learning of the ways they treat their customer base has not endeared me to open one.