From "The Happiness Advantage," quoted (in a great post) by Eric Barker, via Michael Galpert.

...when MIT researchers spent an entire year following 2,600 employees, observing their social ties, even using mathematical formulas to analyze the size and scope of their address books and buddy lists, they found that the more socially connected the IBM employees were, the better they performed. They could even quantify the difference: On average, every e-mail contact was worth an added $948 in revenue.