I think this is a general problem. If Facebook is optimizing for whatever users "Like", as you suggest with the data scientists, it is analagous to a business given customers whatever they ask for.
As Henry Ford is said to have said, "If I had asked people what they wanted, they would have said a faster horse" (or something). You have to find a balance between "the customer is always right" and the fact that you are in a better position to be a domain expert.
They have to balance "what gets people engaged" with "what's good for everybody" and hopefully find an optimal solution for both.
Not sure how accurate it is, but the way Facebook's founding ideas are portrayed in The Social Network definitely screams "exploiting human psychology".