City is a complex combination of physical space and dynamic human behavior. With the popularization of smart-phones, universal coverage of the 4G Internet and advancement of 3S technology, social media hits the main stream in citizens' daily life, and responsively serve as a valuable database to conduct urban research. This paper introduces several urban studies using social media, and has sorted out three main data categories from user-generated content on social networking: locations, photos and opinions/descriptions, which contribute to related urban studies separately or in combination. This new research instrument covers larger scale, reveals real-time activities, and reduce cost, compared to traditional research approaches. Potentially, social media will serve as a vital research database and a tool to study the present and the past of a city for urban planners and decision makers.