In recent years, the impact of Korean pop (k-pop) idols has hugely changed the ecosystem of the western entertainment industry. Online communication among k-pop fans has been playing an essential role in this promotion. This project explores such interactions among the fandom communities of idol groups from the top companies (SM, JYP, YG, and HYPE) on Reddit. Motivated by Tan’s idea of community genealogy, this research captures the emergence of new fandom subreddits by tracing the posting history of founders, the first k members in a new community. The genealogical structure among the fandom groups is represented as a weighted directed network. Based on this network, this research characterizes the member flows between parent and children subreddits. An interesting finding is that while the inclusion of newcomers tends to balance the distribution of a subreddit’s members in terms of the generation and company affiliation of their interested idol groups, it intensifies the homophily based on their idols’ gender. Besides, the study shows that strong connections to relatives, diversified founder backgrounds, and founders’ active involvement (i.e., posting frequency) contribute to the growth speed of a fandom community.