Both asynchronous and synchronous discussions have advantages and limitations for online learning. This study conducts an empirical analysis of these discussion activities while applying the proposed Seamless Online Learning Integrated Discussion (SOLID) system, which can instantly integrate Facebook's asynchronous discussion function with the MSN Messenger's synchronous discussion function. This study examines two teaching activities (a problem-solving-based activity and a project-based discussion activity) within the SOLID system and compares their differences in knowledge construction phases and cognitive dimensions via a quantitative content analysis and lag sequential analysis. The results show that both cognitive and knowledge construction dimensions exhibit a higher diversity of interaction in the project-based activity than in the problem-solving-based activity. In addition, based on the results of the research, this study offers, as reference, recommendations to instructors and researchers for the implementation of online discussion activities.