This study was conducted to see the effectiveness of using Facebook for expressing opinions on given topics to improve students' oral proficiency and critical thinking skills in an English class at a private university. The participants were 80 students enrolled in a course which emphasized the use of English for expressing ideas. Three research questions were formulated to evaluate the effectiveness of teaching and learning. Data were collected by two small group discussion tests, a questionnaire and postings in Facebook. The results indicated that students achieved better performance in the second small group discussion test. There were positive correlations between students' final score and their participation while satisfaction was not found to be correlated to both final score and number of postings. Also, they had a high level of satisfaction with the discussion activity they did in Facebook. The findings suggest that different learning activities be included in future courses to allow students to practice more on discussion, making them improve not only their critical thinking, but language skills as well, and in evaluating those activities, we need to take students' participation into consideration.