The distinguished features of mobile computing bring both opportunities and challenges to the development of Mobile Social Networking (MSN) applications. Learning the successful experiences from existing MSN applications is valuable to both industry and academy. The paper aims at identifying the success factors of Instagram, a well-known MSN application. By performing a historical case study, rapid development and reaction, continuous focus on system scalability on demand, and creating a good external environment are regarded as the three important success factors. To achieve those success factors, 1) using existing mature Open Source Software (OSS) extensively and consulting resident experts at websites when necessary are the secrets to develop and react to new requirements rapidly; 2) monitoring the status of the system by tools and focusing on scalability in both architecture design and detail optimization are vital to guarantee the system scalable on demand; and 3) creating appropriate social networks for different type of stakeholders using right popular social media tools is important to create a good external environment effectively. Based on these insights, the development methodology of Instagram has been extracted, that can be used to guide the MSN application development. At the end, the validation, reliability and generalizability of the information gathered from the case study are discussed thoroughly.