Social media has been recognized as a promising online communication environment that supports self-directed informal learning activities outside educational institutions. Development of metacognition is necessary for self-directed learning. Nevertheless, most studies have focused on the use of social media for formal learning. To fill this gap, we consider the specific nature of informal learning on social media and conduct an empirical study targeting individuals who initiate informal learning on one of the most popular social media platforms in China (Bilibili). We derive a new term, Metacognitive Involvement (MCI), to consider multiple facets of metacognition and examine how various MCI patterns change over time and their associations with social interactions based on Social Network Analysis (SNA). In total, 2,434 comments are manually coded and analyzed from one of Bilibili's most popular learning channels. Our findings reveal that unlike in formal learning where learners do not divulge regarding metacognition, in an informal learning environment they are more active in sharing MCI across time. Furthermore, MCI plays the role of, among others, triggering interactions and MCI-related patterns. It highlights that informal learning on social media is turning MCI development from a static reflection by individuals to a highly dynamic and ongoing process impacted by others. The implications of this study are related to a further understanding of learners' MCI development as well as how the dynamic mechanisms of informal learning on social media could promote personal development.