ABSTRACT Purpose Using social media (SM) in the workplace is controversial. Managers are concerned about boundary spanners’ heavy use of SM for unproductive activities, while boundary spanners need task-focused SM usage to enhance cooperative intention, share mutual knowledge, and build long-term relations. To clarify the effects of SM usage, the authors draw on task–technology fit (TTF) theory to examine how task-focused SM usage (TSM) and relationship-focused SM usage (RSM) influence the efficacy of interpersonal guanxi in China’s emerging economy. Methodology/approach The authors test the proposed theoretical model by collecting 550 valid questionnaires of survey data from manufacturers in China. Ordinary least squares regression is used to analyze the data and test the hypotheses. Findings The results of the statistical analysis show that interpersonal guanxi improves interfirm coordination and exchange performance. Furthermore, TSM usage strengthens the positive impact of interpersonal guanxi on interfirm coordination but weakens the positive effect of interpersonal guanxi on exchange performance, while RSM usage enhances its positive impact on exchange performance but weakens the positive effect of interpersonal guanxi on interfirm coordination. Research implications This study contributes to the literature by developing and testing the TTF-based framework of interpersonal guanxi and SM usage. The finding enriches our knowledge on the roles of interpersonal guanxi in the business-to-business (B2B) context. Practical implications This study offers the guidance for firms using SM in the B2B context. SM usage should be fited with firms’ and boundary spanners’ tasks, considering the complementary and supplementary nature of SM usage. Originality/value/contribution This study tests the different roles of SM usage, i.e., TSM versus RSM, in fostering the efficacy of interpersonal guanxi, providing a meaningful perspective on firms’ knowledge activities. Moreover, by proposing a TTF-based framework of interpersonal guanxi and SM usage, it explores the informational roles of boundary spanners’ SM usage in promoting dyadic relationships.