As a primarily synchronous social media form, social live streaming services (SLSSs) offer real-time interaction between streamers and viewers, and among viewers. Users' value cocreation has become increasingly crucial for platform businesses to increase their competitive advantage. However, the previous studies using the microfoundations approach have only confirmed the employees' efforts to adopt technology as a way to achieve the firms' goals. This study explores the microfoundations of external actors' (viewers and streamers) value cocreation on sports live streaming platforms (SLSPs). Taking China Sport as a case study, this paper conducted netnography research with observations made of four live-streamed matches on the final matchday of the International Table Tennis Federation (ITTF) World Tour Grand Final 2019. In total, 16,204 real-time messages and 5,540 gifting messages were reviewed. Indepth interviews were also conducted with five streamers and 15 viewers. As a result, a typology of viewers (managers, fans, and audiences) emerged, and five viewer-streamer-viewer value co-creation activities were revealed. Furthermore, the unique value-in-use among streamers and viewers in different activities was found. This study presents a model to shows that the viewers' engagement and the value co-creation activities between viewers and streamers at a microlevel determines the value-in-use formation, which in turn, contributes to the competitive advantages for SLSPs at a macro level. This study contributes to the existing literature of engagement behaviour and value co-creation by empirically examining the role of external actors' engagement as the microfoundations of value co-creation in the context of new social technologies -SLSPs.