Research is being conducted to understand human social interactions on the Web as a biological ecosystem. Keystone species in a biological ecosystem are defined as species that significantly impact the ecosystem if removed, irrespective of its low biomass. Identifying keystone species is an important issue, as they play a vital role in maintaining the entire ecosystem and its biodiversity. We hypothesize that a Web system is akin to an open, living ecological system that evolves and sustains itself by constantly updating its elements, which are sustained by the emergence of keystone species. We use data from an online bulletin board and identify keystone threads (”species”) that have a large impact if they are removed or become unpopular, despite their small population size. Our analysis confirms that keystone threads do exist in the system. The system seems to asymptotically evolve to a critical state. At the same time, the number of keystone species increases, and metabolism is enhanced. From a network topological perspective, the system evolves into a network with a high degree, closeness, and PageRank centralities. These findings suggest that keystone species play an important role in the evolution of online ecosystems. Further, by having keystone species, the system itself can decrease stability and bring about diversity to the ecosystem; consequently, the system can evolve.