Let me begin by saying that it's sort of fun to see social science getting so much attention! :) This is broadly my area of research (computational social science and online communities), and this is somewhat similar to the kinds of research I have done, and would like to do in the future.
So, naturally, I think that this sort of research is OK to do. I'll leave out the question of informed consent for now (although it's certainly important), and give a few reasons why I think that this sort of research is important and should be done.
- Websites provide a great way to study the understudied. For years, the main source of our knowledge about human psychology has been college undergraduates. Facebook users provide a much, much broader group of participants (although certainly still skewed toward Westerners).
Using these sites as a context for research should allow us to be more representative in the way we understand the world (and therefore in how we create policy based on that research).
Critics of this research underestimate our autonomy. The press around this has acted as though Facebook's experiment was causing suicides, and creating chaos around the world. This is similar to the "magic bullet" theory of communication. Some early Communication scholars thought that messages from the media could directly influence people.
In reality, messages from the media (or our friends) combine in a very complicated way, and we choose how to respond and react. Indeed, the effect of the Facebook study was just barely statistically significant - people were barely, barely more likely to post positive things when reading positive things, and barely, barely more likely to post negative things when reading more negative things.
- This leads to my next point. There are opportunities for real insight. My wife has been incredibly supportive of my career, but she studied genetics, and doesn't appreciate the social sciences as much as I do. She often says that the results of social science are obvious. I argue that the results seem obvious in retrospect, but that the opposite result would also seem obvious.
For example, one of the arguments against this study is that Facebook was intentionally making people sad by showing them more negative posts. However, previous research actually suggested that when people saw lots of positive posts they became more sad - they would see all the great things their friends were doing, and become depressed by comparison. The Facebook study showed that this isn't true - an important finding.
This is related to my main point. We are spending more and more of our time on these types of sites. We need to understand them. Facebook and other sites have every right (IMO) to run these sorts of experiments in order to improve the experience of users. We can either encourage them to publish their findings, and make their insights available to the public, or we can criticize their research, and encourage them to keep it private, and manipulate our behavior without our knowledge.
I prefer the former.