Recent research demonstrates that it is the quality rather than the frequency of social networking experiences that places individuals at risk for negative mental health outcomes. However, the mechanisms that account for this association have yet to be examined. Accordingly, this study examined whether the tendency to negatively compare oneself with others while using Facebook leads to increases in depressive symptoms, and whether this association is mediated by increases in rumination. A sample of 268 college-age young adults completed an initial online survey and a 3-week follow-up. Path analysis was used to test the hypothesized model, wherein negative social comparison on Facebook was predicted to be associated with increases in rumination, which, in turn, was predicted to be associated with depressive symptoms. The model controlled for general social comparison to test the specific effect of social comparison on Facebook over and above the tendency to engage in social comparison in general. Results indicated that the hypothesized mediation effect was significant. In sum, in the context of social networking, negatively comparing oneself with others may place individuals at risk for rumination and, in turn, depressive symptoms. Findings increase understanding of the mechanisms that link social networking use to negative mental health outcomes and suggest a continued emphasis on examining the specific processes that take place in the context of social networking that may be pathogenic.