The fact is that the experiment they Facebook conducted was mild to what other corporations do every day under the umbrella of "marketing".
They use control groups and try every trick they can to manipulate your mood, feelings, impressions of their products. They carefully script interactions to take advantage of your feelings and social norms. Also take the recent example in the past few weeks of the scripts that Verizon's 'account retention' departments use to try and wedge people into keeping their account longer. Those weren't just thrown together, those were made with careful research and years of experiments on customers and focus groups.
The only difference with what Facebook did and the rest do is that they shared their results with everyone. Was Facebook Unethical manipulating people the way they did? I think so, and I'm only less interested in the service after that scandal, but what they got them in trouble was sharing it with the rest of the world in a way that might have also done some honest good. Now they will learn from their mistakes, keep it to themselves, and use that research purely to manipulate people for higher profit and no one will say a thing.