The benefits of using social annotations to promote collaborative learning experiences have been investigated in the context of digital reading and listening, while its implications in multimedia learning contexts remain unclear. To investigate the influence of social annotations on multimedia language learning, we invited 16 African participants who were learning Chinese as a second language through a MOOC platform in China. They were given full autonomy to study and to shift instructional videos between three modes: no visual aids, linguistic captions, and social annotations. Guided by self-determination theory, the qualitative data were collected through one-to-one interviews, while quantitative data, such as the duration of watching and the number of questions raised by participants, were obtained from observations. Statistical analysis suggests that MOOC learners demonstrated a higher level of engagement and motivation in the social annotation mode than in the other two modes. These statistical findings can be explained from the following three perspectives: content and emotion-related social annotations by other L1 viewers can assist in (1) checking their comprehension of the video by comparing their understanding with others; (2) adjusting learning goals at both the linguistic and content level and therefore using self-regulated learning strategies to a greater extent, especially in actively seeking assistance and other learning resources, and (3) relating the video content to the teaching curriculum/personal experience/learning goals, therefore creating a sense of belonging to a larger language learner community as language learners, interpreters and contributors to the video social annotations.