A social network is a set of people, organizations, or other social entities connected by a set of social relationships, such as friendship, co-working or information exchange to augment social network. While the traditional forms of social networks are often constrained geographically, the emergence of social information infrastructures mediated by information and communication technologies provides a new way to form social networks as digital cities with potentially unlimited boundaries. with a case study, this study attempts to apply the social network concepts and design principles of digital cities to promote and structure social interaction among the users of the digital cities, to stimulate commerce and collaboration activities, to promptly disseminate linkage between discovery and societal benefits of knowledge and wealth creation, and consequently to enhance members' business networks. Advantages, design principles, and future directions of social network based digital cities will also be presented.