Abstract: This study explores the structures of follower/identity networks among environmental nongovernment organizations on Sina Weibo by using social network analysis techniques, and unpacks the formation mechanisms of these networks by integrating the literature of interorganizational network, social movement coalition, Internet studies, and the institutions of social organization management in China. In particular, this study proposes an appropriateness principle to explain the effect of registration status on network structure. Descriptive network analyses show that close virtual relations exist among environmental NGOs, and reciprocal relations are prevalent. Results of exponential random graph models show that the formation of virtual relations is influenced by selforganization mechanisms, organizational resources, appropriateness principle (registration status), homophily principle (offline collaboration relations, geographical location, and focus areas) and activity levels of Weibo use. Organizational resources function as a “signal” on which users base their evaluation of the trustworthiness of the organization, and thus organizations with unknown funding are less likely to be followed or identified with. With regard to the organizations whose funding is known, those with lower levels of resources tend to use the Weibo platform more actively. Unregistered organizations tend to actively build relationship with others, while registered organizations tend to avoid establishing relationships with the unregistered. These findings highlight the importance of legitimacy and lend support to the appropriateness principle. Nongovernmental organizations which have offline collaboration relations or reside in the same province are more likely to form follower/identity relations, and those with similar focus areas are more likely to form identity relations. Activity of Weibo use also has a positive impact on the formation of interorganizational relations. Overall, findings of this study suggest that “low cost of internet use” alone does not provide a sufficient explanation of interorganizational relations on the cyberspace. The authors argue that on a highly interactive social media platform like Weibo, trustworthiness of an organization and its capacity to earn recognition from peer organizations plays a crucial role in the formation of interorganizational networks.

Key words: inter-organizational network, , environmental NGO , Weibo , homophily principle , appropriateness principle