In online learning at scale, wherein instructional videos play a central role, interactive tools are often integrated to counteract passive consumption. For example, the forum or discussion board is widely used, and an emerging functionality, danmaku, which enables messages to be synchronized with video playback, has also been utilized recently. To explore how mass participation is accommodated and what categories of interaction learners implement, this study utilizes analysis of interaction and manual content analysis through learner-generated text data from two specific tools employed in a massive open online course (MOOC) setting: the discussion board (N = 739) and danmaku (N = 2435). Results of the analysis of interaction indicate that mass participation is managed differently by the tools: danmaku fosters a collective space for massive participants, while the discussion board organizes them into threaded small groups. In addition, results of the content analysis show danmaku primarily supports indirect interaction with a focus on the socio-emotional dimension, while the discussion board serves as a platform for direct discussions, particularly in the cognitive dimension. Furthermore, within the context of large-scale engagement, various levels of joint interaction, in addition to collaboration, are discerned and discussed in both socio-emotional and cognitive interactions. The findings offer insights for developing sociable and scalable socio-technical environments in computer-supported collaborative learning (CSCL), addressing emerging educational trends. Practical implications for educational design based on these findings are also discussed.