This is a great point and too seldom raised when comparisons like this are made. I only have so much capacity for highly focused engagement in a given day, and

This is a great point and too seldom raised when comparisons like this are made. I only have so much capacity for highly focused engagement in a given day, and even within those periods there is variability to my focus. That Facebook, or any other activity, occupies time that I am less engaged is not an indictment of those activities or the choice to spend that time in a less deliberate way; its just how most people work. A better criticism in my mind would be to weigh Facebook and its value against actual competitors for people’s attention in those time periods.