facebook's business is advertising, and the more time people spend on facebook, the more valuable facebook's ad space is. If depressed people spend more time on facebook instead of getting out, then it's in facebook's interest to have their product make people depressed. If angry people spend more time writing rants on facebook, then it's in facebook's interest to make people angry. I may be confusing causation with…
If your scientific test shows you that feature set A has better user retention than feature set B, you don't need to worry yourself over why.