Before revision:  We show the expected social welfare is monotone but neither submodular nor supermodular . Nevertheless, we show that a simple greedy allocation can ensure a (1-1/e-\epsilon) -approximation to the optimum .
After revision:  We show the expected social welfare is monotone but neither submodular nor supermodular , surprisingly a simple greedy allocation can ensure a (1-1/e-\epsilon) -approximation to the optimum .