Purchasing decisions are often strongly influenced by people who the consumer knows and trusts. Moreover, many online shoppers tend to wait for the opinions of early adopters before making a purchase decision to reduce the risk of buying a new product. Web-based social communities, actively fostered by E-commerce companies, allow consumers to share their personal experiences by writing reviews, rating reviews, and chatting among trusting members. They drive the volume of traffic to retail sites and become a starting point for Web shoppers. E-commerce companies have recently started to capture data on the social interaction between consumers in their websites, with the potential objective of understanding and leveraging social influence in customers' purchase decision making to improve customer relationship management and increase sales. In this paper, we present an overview of the impact of social influence in E-commerce decision making to provide guidance to researchers and companies who have an interest in related issues. We identify how data about social influence can be captured from online customer behaviors and how social influence can be used by E-commerce websites to aid the user decision making process. We also provide a summary of technology for social network analysis and identify the research challenges of measuring and leveraging the impact of social influence on E-commerce decision making.