Social Networking Sites (SNS) have a high penetration rate among adolescents who are considered as the Net Generation. Many studies have shown that SNS can increase students' motivation to learn. Therefore, more and more instructors are trying to incorporate SNS into their delivery channels in order to better their student's learning outcomes. However, whether or not SNS effectively help students elevate their EFL learning performance still needs further investigation. This study plans to explore how EFL learners perceive SNS. A Facebook-based community of practice (CoP) was engineered by a group of English majors where English was the only language used. The study reports on a 3-month action research that monitored the EFL learners' engagements in Facebook's community of interest. The participants were forty-three English majors (N = 43), freshmen from a university in southern Taiwan, who have been actively using Facebook for more than a year. The results of the study presented an interesting aspect of the students' engagements in the activities, which were greatly influenced by the nature of the instructional activities and the feedbacks posted by their peers. Furthermore, results of an AHP analysis pointed out that these EFL students preferred involvement in instructional activities via the Facebook platform. They valued the criterion of interactivities on Facebook, with the greatest weight obtained from an AHP.