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. 

 There's also this: https://unlimitedhangout.com/2021/04/investigative-reports/the-military-origins-of-facebook/