Device-to-device (D2D) communication has been proposed as a promising technology for future cellular communication systems due to its advantages of high spectrum efficiency, low energy consumption, and enhanced system capacity. Resource allocation for D2D communications, which occupies nonorthogonal channels with cellular transmissions, is an important problem in terms of achieving the aforementioned benefits. In this problem, there are two fundamental challenges to be addressed: 1) how to emulate cellular users to share their resources and 2) how to efficiently allocate resources in terms of channels for D2D pairs. In this paper, we exploit social ties in human-formed social networks to enhance D2D resource sharing and further propose a social-community-aware D2D resource allocation framework, where cellular users would like to share their channels with D2D communications in the same community formed by a group of people with close social ties. After that, we propose a two-step coalition game, where a coalition formulation is established for communities, and an optimal resource allocation problem is formulated for D2D pairs. Extensive simulations on random networks and real mobile trace verify the effectiveness of the proposed scheme.