Recently many companies have featured their applications as SaaS (Software as a Service) applications where applications will be treated as services and provided online for thousands and millions of users. Social Networking SaaS (SNS) is one of the most popular kinds of SaaS. The key to the success of a SNS heavily relies on the scale of users. With the explosive growth of users, SNS's architecture should be able to change according to the demand. To better understand what kind of requirements influences SNS's architecture most, and learn the relationships between these requirements and the evolution progress of architecture, this paper carried out a dedicated analysis on a popular SNS application, Instagram. The lessons learned show that: 1) Demands on scalability and real-time are the main driven forces to architecture redesign, 2) Data coming from extensive monitoring is one of the most important sources of evolutionary requirements/issues, which drives different types of architecture evolution, 3) Reusing existing components is the key factor to evolve architecture rapidly and costly. The lessons learned can help engineers and researchers understand the co-evolution progress between requirements and architectures, and how to devise appropriate architectures to meet the possible evolution challenges for SNS.