Social networking service (SNS) is known for its ability to facilitate people's interaction and build connections. Recent research also showed students' interest in learning activity using SNSs. Besides, few studies have also investigated the effect of adopting SNSs in learning activity. However, studies that examine knowledge construction behaviors in asynchronous online discussion using SNSs in adult and continuing education are quite limited. The purpose of this study is to explore the knowledge construction behavior in an art course which involved a project-based learning activity using a popular SNS, Facebook, as a discussion tool. Participants are comprised of 62 students in the school of continuing education of a university in northern Taiwan. The results show that students primarily share knowledge and information when discussing online. The diversity of knowledge construction is limited in the context of this study. In addition, further analysis on the relationship between students' individual differences and knowledge construction and the extent of participation suggests that there is a gender difference in and correlation between age and knowledge construction behavior. Implications and limitations are discussed while pedagogical suggestions for instructors using SNS as an online discussion tool for teaching activity in adult and continuing education are also proposed.