Students' extensive use of Facebook in their daily lives has led researchers to investigate the affordances of Facebook for educational purposes. To further the research into the use of Facebook to improve language teaching, we conducted a convergent parallel mixed-methods study to examine the use of Community of Inquiry-informed Facebook discussion activities on the speaking performances of undergraduate students in a blended EFL speaking class in Bangladesh. A Facebook group was maintained for both the treatment and control conditions; however, the discussion activities were required only by the treatment condition. We found a statistically significant difference between the initial and post-test speaking scores for the treatment and control conditions. While no difference was observed in post-test scores between the two conditions, students' and the instructor's comments on the Facebook group and student interview data revealed that Facebook was helpful for both conditions in improving their performances, but in different ways.