Discussion forums are the primary and most widely used venue for social learning and communication in MOOC context. There has been an interest in the last few years to study the interactions and participation behavior in this environment. This systematic review studied eighty-four papers published between 2013-2017 to provide a state of the art, and suggest future directions in the literature from two perspectives: descriptive analysis and content analysis. The descriptive analysis aggregates the reviewed papers into five dimensions: time of the publication, stakeholders involved in the analysis, objectives, methods, and data sources used in the studies. The content analysis aims to highlight the main issues addressed and the major contributions of the papers covered by the literature review. These contributions can be categorized into three major areas, first, explore the relationship between forums participation and retention or learning outcomes and evaluate ways to increase the participation activity and learning outcomes, second, analyze participants' contribution in the forum and how to organize the content and efficiently monitor them, finally, analyze participants' interactions and how it influences learning. At the end, we offer some suggestions to further advance this area of research.