Without properly addressed, the dramatic increase in mobile data traffic may lead to severe traffic congestion problems in cellular networks. Mobile data offloading is a promising method to reduce the traffic congestion by redirecting the traffic of a cellular network to other types of access networks. In addition, with the popularity of online social platforms (e.g.,Wechat and Facebook), the social relationship of mobile users (MUs) has become an important factor to impact the decision of data offloading. In this paper, we take the social relationship of MUs into the consideration of incentive mechanism design and design two social-aware incentive mechanisms to achieve efficient mobile data offloading, in which a PageRank-based algorithm is used to model the social relationship of MUs. The proposed incentive mechanisms satisfy the properties of individual rationality and truthfulness. Extensive simulation results demonstrate the nice performance of the proposed mechanisms by comparing with other counterparts.