Social media, whose core element is interactivity, has become an important tool for people to establish and maintain relationships. According to social exchange theory (SET), weighing costs and benefits in the context of online interpersonal interactions can also guide people to decide whether to make a connection with others. While there have been studies of social interaction, few have looked at specific behaviors, including sharing and responding, within the same framework. Therefore, this study aimed to investigate the benefit-cost balance of social interaction behaviors on social media that involve self-disclosures and proactive interactions such as commenting, forwarding, and liking. Via an online survey of Chinese adults ( n = 2,767) and structural equation modeling (SEM) techniques, we found that social media users were at once motivated by bridging social capital and restrained by privacy concerns when making decisions regarding self-disclosure. However, users were not swayed by privacy concerns when engaging in proactive interactions; instead, they were encouraged by both bonding and bridging social capital. Moreover, the benefit-cost model was extended by investigating the moderating effect of attachment anxiety and found that the more anxious individuals were about attachment, the weaker the effects of bridging social capital and the stronger the effects of privacy concerns on their social interaction behaviors, which complemented the existing literature at the level of individual differences.