If you're prepared to toss ethics overboard it's not that hard to come up with significant, relevant and valid results. The whole trick is to get those results while acting ethical.
Where's the ethical concern here? Familial relationships can be explicitly provided as can occupation. If it's all self-reported data then what's the problem?
The problem with the emotion manipulation study was that they were effecting people's psychology, not just click behavior. Which is creepy, especially because it apparently worked.
I find it interesting that you don't see the problem. I suspect that is a large part of why companies like FB and the various advertising networks are utterly blind to what will eventually be their downfall.
Here is a nice example of a sample consent form the kind of which one would expect to fill out before having one's data be used for any kind of study:
Not being informed about the fact that your data is going to be used in a study, that the results of that study are going to be published is un-ethical in several ways, for one it presumes consent where non exists, for another it is a breach of the law (because it violates the original purpose of collection). Not that Facebook would care about any of the above but companies and institutions that play by the rules would require the researcher conducting this study to obtain consent from the subjects.
Consent is given through the click-through user agreement and the data policy https://www.facebook.com/policy.php . It's no different, and anything more is impractical. There are literally hundreds if not thousands of tests being done simultaneously on overlapping user populations.
I think there's another issue that being willing to be completely unethical requires such distorted thinking (for humans, anyway) that it introduces its own sort of spurious results. Just thinking about a lot of callous and inhumane psychobabble that was promulgated not too long ago, mostly dealing with people whom the researchers thought of as not-quite-as-human.
Indeed, that's another risk. But for now FB seems to have steered clear from research in that vein. Even so I can't read about FB conducting 'studies' without thinking of this:
The problem with the emotion manipulation study was that they were effecting people's psychology, not just click behavior. Which is creepy, especially because it apparently worked.