Increasingly more devices are using WiFi Direct to establish direct, peer-to-peer connections to each other. By leveraging opportunistic communications through WiFi Direct, mobile social networks (MSNs) enable people with mobile devices to share information without the support of Internet connection. However, the half-duplex nature of WiFi Direct does not allow connected nodes to transmit data simultaneously over the connection. In addition, the contact duration among mobile nodes in an MSN tends to be short due to their mobility. As a result, they may not be able to exchange all information as they intend to. In this case, how to schedule the transmission of contacting nodes during the limited contact duration becomes a fundamental issue. In this paper, we design a contact-duration aware transmission scheduling scheme that allows nodes to transmit data in a fair manner. First, we propose a duration allocation problem, that is, how long time each node should be allocated to transmit data within a predicted contact duration. This problem is addressed using Nash bargaining solution. Second, we formulate a fractional knapsack problem to decide which data to send during the allocated transmission time. Third, based on the theoretical results, we design a frame-by-frame scheme to schedule the transmissions of nodes. Finally, we evaluate the performance of our contact-duration aware transmission scheduling scheme through simulations.