They were doing a study on how manipulating the algorithm could result in a change in users' emotional states
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Sounds evil when it's put that way. But, consider what practically every business does every day with A/B testing. Literally playing on your emotions to buy. Not defending FB, but they sure do get a lot of focus when everyone is basically doing the same thing.
That's very fair. The scale is probably the thing that makes it so clearly unethical because you're not casting a line into a crowded pond anymore, you're competely changing the environment the fish swim in.