MOOCs, being a new education paradigm, present both opportunities and challenges to educators and learners. On one hand, their massive and open nature allow learners from all over the world to take part in learning their favorite subjects in a flexible manner. On the other hand, the lack of face-to-face engagement also introduces a sense of isolation in students, resulting in a high dropout rate. To this end, online forums are deemed to help overcome the problem by enabling interactions among the learners and facilitating their collaborative learning. Recent research further suggests that with an online forum provided in a MOOC, the presence of instructors may not matter. However, in this study, with an examination of individual-level behavioral data and how much the learners persisted in assignment completion, we provide two important insights: First, online forum indeed plays an important role in motivating learner continuance, in that those who visited the forum completed more assignments. Second, among the different types of forum activities, including visiting, posting, interaction with peers, and interaction with instructors, we find that interaction with instructors emerge as an important factor contributing to more assignment completions in addition to forum posting. Thus, although the importance of online forums in MOOC is re-affirmed, our research cautions the view that the presence of instructors is trivial in MOOCs when online forums are provided. Implications for research and practice are discussed.