I. Introduction
Motivation. Online social networks, such as Facebook and Myspace, have experienced an explosive growth recently. Mobile social networks, which bring social networking to mobile phones, represent a natural next step and have already generated a lot of excitement. For example, cell phone manufacturers and cellular service providers have developed their own social networks (e.g., Nokia, Virgin Mobile) and provided software support for mobile social networks (e.g., Motorola). Compared to (Internet-based) online social networks, mobile social networks offer several distinctive advantages: (i) much larger potential user base, with 4 billion mobile phone subscribers [33] compared to only 300 million broadband subscribers [7], (ii) the built-in localization capability of mobile phones which enables rich and new location-based services, and (iii) applicable to a wider range of application scenarios because unlike online social networks, they do not necessarily require access to a computer or the Internet.