College students are the prominent users of social networking sites. Some studies revealed negative effect of social networking on academic performance, so finding some variables which can mediate this negative effect is important. This descriptive-correlational study examined the mediating role of informal learning and engagement in relationship between social networking and academic performance. To this aim, 380 university students were selected using cluster sampling. Social Network Sites Usage, Student's Engagement in School Scale, perceived educational outcome of social networks and student self-reported GPA were used as instruments. Findings from path analysis revealed that the direct effect of social networking on academic performance and engagement was negative but its impact on informal learning was positive. Path analysis showed neither informal learning nor academic engagement can mediate negative effect of social networking on academic performance.