In this paper, we address the energy-efficient contact discovery issue in Pocket Switched Networks (PSNs), in which the nodes' mobility pattern shows strong social property. In a PSN, although the end-to-end connection may be disconnected most of the time for a given source-destination pair, several nodes periodically gather at certain hot spots and form well connected clusters. Based on such mobility pattern, cooperation among nodes is utilized in our contact discovery design. The nodes that have already joined a cluster collaboratively wake up to discover new contacts. With local synchronization, the nodes can be operated in sleep mode more frequently, leading to high energy efficiency. Both the theoretical analysis and simulation results show that our cooperative duty cycling (CDC) greatly reduces the energy consumption while achieving comparable data delivery performance.