Online social presence supports student learning by making group interactions more appealing and has become a central concept in computer-mediated communication. However, questions remain over how social presence is presented in a mobile instant messaging (MIM)-facilitated environment and to what extent MIM can afford social presence compared to a threaded discussion forum. This study offers a new contribution by examining the social presence levels afforded by a MIM app (WeChat) and comparing it with a threaded discussion forum. We present a two-stage study. In stage 1, we analyzed social presence levels in the MIM postings of class A with a validated instrument comprising three dimensions, namely, affective, interactive, and cohesive responses. In stage 2, we employed a historical cohort control experimental research study to compare social presence levels manifested in class A to those in class B who used an online forum. Follow-up interviews were conducted to solicit explanations of the differences in social presence levels. The results show that compared to the asynchronous threaded forum, the quasi-synchronous MIM is particularly suited to promoting (a) expression of emotions (affective social presence), (b) expressing agreement (interactive social presence), and (c) phatics and providing support (cohesive social presence). Four attributes might contribute to the differences including (a) the ease of use of non-verbal cues, (b) location-free digital interaction, (c) presence awareness, and (d) multimodality. The implications for future practices and research are discussed.