What are we choosing not to spend time on to make room for the hours we all spend each week on Facebook, Twitter, Pinterest, and the like? While numerous articles have worried that such screen time might be coming at the expense of face-to-face socializing, a new paper from the National Bureau of Economic Research suggests, happily, that that is a relatively small part of the tradeoff. But managers won’t find much comfort in the study’s conclusions. More than anything else, the leisure time we spend online comes at the expense of work.