Date offloading is an efficient way to address the mobile traffic congestion by redirecting some traffic of a cellular network to complimentary networks. Meanwhile, multicast applications (e.g., video conference, IPTV, and the bulk software upgrade) are very popular. However, few of existing work investigates incentive mechanism design for multicast applications in mobile data offloading. In this paper, we focus on the multicast incentive mechanism design and propose a social-aware multicast incentive mechanism (SAMIM) for data offloading, where the cellular operator will offload the data of a multicast group to some WiFi access points (APs), then these APs serve mobile users (MUs) within their coverage area through multicast transmission. Numerical simulations demonstrate the good performance of SAMIM by comparing with IM-only. In specific, SAMIM improves the system social welfare by 20% compared with IM-only with the number of selected APs |W| = 15.