The convergence of social networking and mobile computing is expected to generate a new class of applications, called Mobile Social Networking (MSN) applications, that will be of significant importance for the coming years. Indeed, MSN enhances the capabilities of more traditional Online Social Networking (OSN) to a great extent by enabling mobile users to benefit from opportunistically created social communities; these communities should be determined not only by common interests or contacts but also by mobility-related context, such as physical location and co-presence. In this paper we precisely define what we intend for MSN applications and overview the primary MSN support solutions available in the current literature and that specifically address the underlying technical challenges, design issues, and emerging middleware guidelines. Our primary goal is to identify engineering design criteria for future MSN middleware solutions, capable to flexibly adapt to different application domains and deployment requirements. To this purpose, we present a novel taxonomy of MSN structures and describe how various existing middleware approaches fit the proposed classification; moreover, the survey takes the opportunity of these descriptions to discuss related middleware design/implementation choices determining specific tradeoffs between expressive power, flexibility, and scalability.