Do you understand how widespread this kind of research is? Literally everyone does this.
The act of publishing can't be the ethical breach -- just focus on the research, what do you think they did wrong there?
One of the main objections I'm seeing from people (in my bubble) isn't that Facebook did this, but that Cornell, UCSF, and PNAS participated in this. Facebook can do this, and while it's unethical it's not illegal. Same goes for manipulative people in your everyday life (let me not tell you about the horrific human being of a girlfriend I once had). The point is that science and the people who purport to carry it out should be held to higher and rigorous ethical standards. If those standards are not met, those people should be excluded from science and their findings ignored. They should not be awarded serious consideration in a journal such as PNAS. That is what is happening here as far as I can see, and while a bit dramatic fashion I think it is correct.
Also, if I may toss my personal interpretation of the research into this... ethics aside, the study is extremely weak, and I honestly don't see how it can be published in such a "good" journal. The effect size was < 0.0001. They hand-wavingly try to explain that this is still significant given the sample size. I'm personally not convinced, at all. Sounds like they needed a positive conclusion out of the study and so they came up with a reason for one. If this landed on my desk for review I would have reject on that alone.
If it's the case they should be held to a higher standard simply because it was academic research, it's seems like a terribly inconsistent position to take. But if true, then FB walked right into it, and all we can do is shrug.
Let's draw an analogy here. Let's say that Google decided to conduct an experiment on its Glass users, and that for a period of a week, Google decided to see if it could alter the mental state of its customers by using Glass to delete or diminish positive social interactions. Let's assume this research was conducted without any kind of consent or knowledge of the customers being experimented on. What's your immediate reaction to that? Still cool? Still harmless and just like advertising and A/B testing? Or, creepy and dangerous?
Like it or not, Facebook does have a special responsibility here. It is, quite literally, the lens through which people see their world.
...What about enticing the maximum number of people to click on your buttons for the maximum amount of time, with no purpose but capturing their eyeballs?
"Do customers prefer the green button or the blue button" is harmless. Psychological testing is NOT harmless and NOT something that anyone with a website can or should do. That is medical research - and as such is strictly controlled precisely because of the real risks and genuine human cost that it can have if conducted inexpertly.
That is a psychological test.
So the nature of the scoring algorithm (counting emotional words) used to measure the impact of a change makes deploying the A/B test suddenly unethical?
You see, that's why there are ethics committees at Universities.