You can read the study itself here, plus a very comprehensive discussion of reactions to the study here.
1. If you intend to publish your research in a peer-reviewed scientific journal, you are expected to have conducted that research with the appropriate ethical oversight. Indeed, the submission process usually involves explicitly affirming that you have done so (and providing documentation, in the case of human subjects research, of approval by the relevant Institutional Review Board(s) or of the IRB's determination that the research was exempt from IRB oversight).
2. Your judgment, as a researcher, that your research will not expose your human subjects to especially big harms does not suffice to exempt that research from IRB oversight. The best way to establish that your research is exempt from IRB oversight is to submit your protocol to the IRB and have the IRB determine that it is exempt.
3. It's not unreasonable for people to judge that violating their informed consent (say, by not letting them know that they are human subjects in a study where you are manipulating their environment and not giving them the opportunity to opt out of being part of your study) is itself a harm to them. When we value our autonomy, we tend to get cranky when others disregard it.
4. Researchers, IRBs, and the general public needn't judge a study to be as bad as [fill in the name of a particularly horrific instance of human subjects research] to judge the conduct of the researchers in the study unethical. We can (and should) surely ask for more than "not as bad as the Tuskegee Syphilis Experiment".
5. IRB approval of a study means that the research has received ethical oversight, but it does not guarantee that the treatment of human subjects in the research will be ethical. IRBs can make questionable ethical judgments too.
6. It is unreasonable to suggest that you can generally substitute Terms of Service or End User License Agreements for informed consent documents, as the latter are supposed to be clear and understandable to your prospective human subjects, while the former are written in such a way that even lawyers have a hard time reading and understanding them. The TOS or EULA is clearly designed to protect the company, not the user. (Some of those users, by the way, are in their early teens, which means they probably ought to be regarded as members of a "vulnerable population" entitled to more protection, not less.)
7. Just because a company like Facebook may "routinely" engage in manipulations of a user's environment doesn't make that kind of manipulation automatically ethical when it is done for the purposes of research. Nor does it mean that that kind of manipulation is ethical when Facebook does it for its own purposes. As it happens, peer-reviewed scientific journals, funding agencies, and other social structures tend to hold scientists building knowledge with human subjects research to higher ethical standards than (say) corporations are held to when they interact with humans. This doesn't necessarily means our ethical demands of scientific knowledge-builders are too high. Instead, it may mean that our ethical demands of corporations are too low.