Web-based social networking services enable like-minded people to collaborate and socialize with each other. With rich sensing and communication capabilities, mobile phones provide new possibilities for enhancing face-to-face social interaction among people who are both socially and physically close to each other. Research challenges arise as how to exploit the characteristics of people's mobility patterns and form a social community with a specific goal in the mobile environment. In this paper, we present SOCKER - a dynamic community creation mechanism based on social-aware broker selection strategies. SOCKER gradually forms a mobile social community by dynamically selecting a broker during each opportunistic encounter, and the selected broker disseminates community creation requests to the encountered users for match-making. Based on real human mobility traces, extensive evaluations are conducted showing that SOCKER achieves high community completion ratio as well as high user social satisfaction, while incurring a small overhead