The wide adoption of social media has encouraged university teachers to consider employing social media as a new e-learning platform. This research aims to find the role of social media usage in promoting effective communication and enhancing group task performance. What is more, to give implications on a better grouping method, we utilized the concept of tie strength (in a typical social network) in the current research. We proposed that the tie strength among university students negatively moderates the relationship between social media usage and communication. The social constructivism theory was employed as a major theoretical foundation, built on which a research model was developed. The model was tested via 135 senior-year undergraduates in one of the diversified classes. The data analysis results revealed that social media usage promotes communication in group; tie strength in the social network group negatively influences group communication. This means, contrary to our natural assumption, the closer the relationship is among group members, the less likely communication would happen in the group. Our research gave implications on pedagogy in that it extended our understanding of the project-based learning in small teams supported by interactions via social media. It also contributed to the grouping strategies of the classes that rely heavily on the group discussion via social media.