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.
Discussion
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.