A mobile social network system is generally complex that it comprises several views, such as data, function, structure, behavior and so on. There are two kinds of approaches to design these views. The multiple diagrams approach for mobile social network systems respectively chooses a distinct diagram for each view. The single diagram approach for mobile social network systems, instead of choosing several separated diagrams, utilizes only a single diagram. We propose the structure-behavior coalescence design method for mobile social network systems based on the single diagram approach to prevent the inter-diagram design inconsistency problems.