Mobilizing the bystanders: How social capital drives user reporting on social media harmful content

User reporting is a critical governance mechanism for addressing harmful content, such as misinformation, cyberbullying, and hate speech, on social media platforms. Prior research, largely grounded in the online bystander intervention framework, has emphasized users' situational responses to specific incidents, paying relatively less attention to the enduring social relationships that embed reporting behavior within platform communities. This study draws on social capital theory to conceptualize user reporting as a socially embedded practice. Structural social capital, reflected in social interaction ties; relational social capital, reflected in identification; and cognitive social capital, reflected in value congruence, are theorized to shape users' sense of belonging, altruistic motivation, and reporting self-efficacy, three more immediate psychological mechanisms that help translate communal resources into reporting engagement. Data from a two-wave online survey conducted across multiple Chinese social media platforms were analyzed using partial least squares structural equation modeling (PLS-SEM). The findings show that social interaction ties, identification, and value congruence are associated with reporting engagement mainly through these psychological mechanisms. This work extends online bystander intervention research by demonstrating how users' social capital in platform communities underpins sustained reporting participation, and it offers practical insights for designing reporting systems that leverage community-based social resources.