Because of the rise of social networking sites (SNSs), social comparisons take place at an unprecedented rate and scale. There is a growing concern that these online social comparisons negatively impact people’s subjective well-being (SWB). In this paper, we review research on (a) the antecedents of social comparisons on SNSs, (b) the consequences of social comparisons on SNSs for SWB and, (c) social comparison as a mechanism explaining (mediator) or affecting (moderator) the relationship between SNSs and SWB. The occurrence of social comparisons on SNSs depends on who uses the SNS and on how the SNS is being used with passive use in particular resulting in increased levels of social comparison. Moreover, social comparison on SNSs may occasionally result in an increase in SWB but typically negative effects are found as people tend to engage in contrasting upward social comparisons. Finally, several studies show that social comparison is a key mechanism explaining the relationship between use of SNSs and SWB and that users with a tendency to engage in social comparison are especially likely to be negatively impacted by SNSs. The dynamic, cyclical processes that result from this pattern of findings are discussed.