Online discussions are widely viewed as a valuable tool for encouraging student engagement and promoting interaction with course material outside of the traditional classroom. Strategies for conducting online discussions vary and are not confined to traditional, university-sponsored learning management systems (LMS). Social media platforms such as Facebook, which provide a variety of social benefits to students, might also represent a viable mechanism for educational information exchange and learning. Our study tested this proposition by comparing the effects of a Facebook-based and LMS-based online discussion forum on students' participation, achievement of learning goals, and overall course performance. Our findings suggest that different forums can affect classroom dynamics and student learning in different ways. While Facebook may be better at fostering student participation and encouraging peer-to-peer dialogue, the university-sponsored LMS may be a more effective tool for encouraging students to develop coherent arguments and apply course content in other contexts. Since this study shows that platform of an online discussion assignment matters, college instructors should consider the benefits and drawbacks of each platform before developing an online discussion assignment. Instructor choice of platform should depend on course content, instructor's teaching preferences, and online discussion assignment goals.