User-generated content (UGC) is a valuable information resource on social media. However, we currently know little about why people are constantly producing digital content. In an effort to gather more contents, many UGC service providers are trying to help users improve their social experience. Through the lens of social network theories, our study illustrates that social influence and social interaction between users will motivate users to contribute contents on UGC platforms. We hence design an empirical study to discover the social factors influencing users' productivity. By collecting data from Flickr.com, we expect to support that individuals' perception of "social comparison" and "social identity" will positively influence users' behavior of content generation. We also assume that social interaction, in terms of tie strength, will increase users' productivity of content.