Many people argue that Facebook's study of how the emotions of it's users changed depending on the emotional content of messages in their facebook feed wouldn't have been approved by the average ethical review board because facebook didn't seek informed consent for the experiment.

A slightly strong form of this argument is that the study is unethical because the users didn't consent to being manipulated and studied in this fashion, and that an ethical review board would have noticed this and nixed it. Which is to say, a slightly stronger argument focuses on the ethics issues which might have been averted, rather than on what an ethics review board would conclude, as the former is an argument in its own right where the latter is an argument from authority.

I don't see an ethical lapse here; when Facebook modulates the content of messages without performing an experiment, informed consent doesn't enter into it. If you can do a thing ethically, calling your act an experiment, or paying attention to how people react, doesn't create an additional ethical burden.

Nor does calling your actions an experiment absolve you of ethical burdens, either. Information is valuable, and from a utilitarian perspective the information may outweigh the costs, but the utilitarian obligation to maximize doesn't end at the point where the scales first begin to tip.