While employees gradually use social media for work purposes, it remains unclear how such a use influences their knowledge sharing and work efficiency. Thus, this study explores how work motivations and communication visibility afforded by social media affect employees’ knowledge sharing and work efficiency. We find that message transparency of communication visibility enhances knowledge sharing and work efficiency but weakens the impact of reputation on knowledge sharing, whereas network translucence of communication visibility increases work efficiency and strengthens the influence of social networking on knowledge sharing. Our findings add to the literature and provide guidance on how to use social media for work.