Excessive use of social network sites (SNSs) can often lead to negative consequences of frequent upward social comparisons despite having the social network platform to present users in a favorable light. However, the existing literature gives little evidence to social comparison related antecedents and consequents of uncontrollable use of SNSs. The present study aimed to investigate the contributions of social comparison to SNS addiction. In Study 1, using a convenient sample in Austria (n = 103), we showed that the tendency to engage in social comparisons of ability (but not of opinion) predicted self-reported SNS addiction over and above the feelings of relative deprivation on social support and status. SNS addiction mediated the relations between social comparison of ability and stress, but not self-esteem. In Study 2, using a broad sample of participants in Austria (n = 500), we replicated the findings observed in Study 1 and showed that contrastive upward social comparison emotions (i.e., envy, depression) mediated the relation between SNS addiction and lower self-esteem whereas the contrastive downward social comparison emotion (i.e., contentment) mediated the relation between SNS addiction and higher self-esteem. Our findings suggest that SNS addiction closely relates to psychological constructs relevant to social comparison, mediates the link between social comparison of ability and detrimental consequences (i.e., stress, well-being) and demonstrate how social comparison emotions relate to both positive and negative associations between SNS addiction and self-esteem.