The number of Massive Open Online Courses' (MOOCs) participants has been increasing over the years but its completion rate is extremely low. Social support/social interaction is one of the key factors that has a huge impact on students' learning motivation in both online and offline environments, but difficult to maintain in MOOCs due to its asynchronicity. Built-in social tools are therefore important in the MOOCs learning context because they can provide opportunities for students to interact with both other students and instructors. Present scoping review focused on built-in social tools in MOOCs and aimed to find out: What theories have been utilised to guide or explain how built-in social tools in MOOCs might support students? What kind of built-in social tools have been applied in MOOCs? What kind of outcomes have been investigated in research that focused on built-in social tools in MOOCs? Seventy articles have been included in this review and we found that (1) the majority of the research did not use any theories or models to guide the study design or explain the findings (2) Forum is dominating in numbers compared to other built-in social tools (3) Most studies focused on observed data such as number and content of posts in the forum, and their relationships with course grade and completion rate. However, no research has focused on how built-in social tools might affect students' belongingness, which is considered to have a significant influence on students' motivation to learn. Suggestions to address the research gaps were given.