In general, A/B testing is studying the website. You're not testing us, you're testing the best ways to convince us to do stuff. On top of that, when users visit your website (which is presumably pitching something to them), they know they're being pitched. They know the website is going to try to convince them to do stuff--click over here, watch this video, sign up for foogadgetmcawesome. Same drill with advertising. Yeah, this commercial with the freaking Budweiser puppy made me cr--er, chop onions--in its attempt to get me to buy beer, but I know they're trying to sell me beer, and I knew as soon as the puppy hit the screen that I'd probably start sniffling.
Another missing component here is user control. I can turn off the commercial and stop playing the game. I'm currently working (in a minor capacity) on the National Children's Study, and our participants can walk away at any time. When the Facebook subjects on the negative side started feeling slightly more sad, they had no idea why, and no clue how to stop it.
Who decides what experiments social scientists get to run on 1/7th of the world's population?
The goal of this study was not to improve the site, though: it was to test a hypothesis about social psychology.
Ignoring the classic "I'm sorry you were freaked out" non-apology here, this response completely misses the point, and tries to re-frame the concept of informed consent into an inconsequential piece of red tape that anxious people worry about.
People were made subjects of a scientific experiment without their knowledge or consent. It doesn't matter that Facebook took steps to make the changes as small as possible; it doesn't matter that the effect on the individual subjects may have been minor; it doesn't matter that the results were interesting. It was an ethical breach, and this tone-deaf response is fairly unsettling.