Recently, Computer Supported Collaborative Learning researchers have become interested in the notion of scripting and assigning roles to students enrolled in online courses, in order to increase their engagement in collaborative activities. This study aimed to investigate whether assigning a role called "social tutor" to a student enrolled in an online university course can promote (1) peers' participation in online discussions, (2) the development of a Sense of Community (SC), consisting of three factors: membership, fulfilment of needs and goal achievement and mutual influence of the individual/group, and (3) effective learning. The participants were 53 undergraduate students who enrolled in an online course, and were divided into two groups: (1) a group of 29 students with a social tutor and (2) a group of 24 students with no social tutor. The results indicate that students' participation as well as the membership factor of SC improved only in the group that the social tutor was present. In contrast, the scores on the other two SC factors improved regardless of the presence of a social tutor. Moreover, the results show no difference between these two groups in terms of students' grades in the final examination. The importance of a social tutor in the design of effective online university courses is discussed.