The right of the subject of an experiment to understand and consent to it has been a key ethical consideration since the famous Milgram experiment, in 1963. Nearly 50 years later, Facebook used their users as guinea pigs in a study where their news feeds were manipulated in order to see how it affected the mood expressed in their subsequent posts. Besides being questionable in terms of experimental ethics, this is about like Google manipulating their search results.
Facebook's brand is about letting you know what's most important in the posts from your friends and family. Manipulating that information for research purposes undermines trust in the brand. More so than the intensive data mining which Facebook (and Google!) already subjects all of their users to.