The issue is clear; if a doctor or psychologist tried this, they would have to get IRB approval. You need informed consent; such laws were passed after psychologists had tried a LOT of experiments on the unwitting public; simluating muggings, imminent death scenarios, etc.
I know people say "it's just manipulating feeds, what's the harm?" There can be plenty of harm if you manipulate the feeds. Where is the line? What if facebook had decided to see what happens if you try showing depressing posts and bad news for a year? Or a feed where you were always ignored? No IRB would allow something like that if it risked permanent mental scarring or created a suicide risk.
Bad move, Facebook. Experiments are definitely cool (I'm a researcher), but we go through proper channels and regulation for a darned good reason.