We study user behavior in the courses offered by a major massive online open course (MOOC) provider during the summer of 2013. Since social learning is a key element of scalable education on MOOC and is done via online discussion forums, our main focus is on understanding forum activities. Two salient features of these activities drive our research: (1) high decline rate: for each course studied, the volume of discussion declined continuously throughout the duration of the course; (2) high-volume, noisy discussions: at least 30 percent of the courses produced new threads at rates that are infeasible for students or teaching staff to read through. Further, a substantial portion of these discussions are not directly course-related. In our analysis, we investigate factors that are associated with the decline of activity on MOOC forums, and we find effective strategies to classify threads and rank their relevance. Specifically, we first use linear regression models to analyze the forum activity count data over time, and make a number of observations; for instance, the teaching staff's active participation in the discussions is correlated with an increase in the discussion volume but does not slow down the decline rate. We then propose a unified generative model for the discussion threads, which allows us both to choose efficient thread classifiers and to design an effective algorithm for ranking thread relevance. Further, our algorithm is compared against two baselines using human evaluation from Amazon Mechanical Turk.