Well if your study involves depressed people unknowingly being manipulated with negative newsfeed items that they didn't know they consented to being subjected to in a secret study, then that's unethical. The scientists aren't stupid, they knew beforehand that in a sufficiently sized population, there will be a segment of depressed people that they are influencing to commit suicide by being subjected to increased negative newsfeed material. That is unethical to me any way you cut it.