Evidence suggests that girls spend much time on social networking sites (SNS), often more than boys do. It has been proposed that this may have to do with sex-based differences in the need and approaches for socialization. We posit that adolescent girls are also unique in that they are developing physiologically and start menstruating. Based on prior research, we hypothesize that the onset of menstruation can drive physiological changes (increased body mass index (BMI)), which together with common behavioral–psychological (social and emotional) responses to menstruation can contribute to an increase in SNS use. We therefore aim to test whether BMI partially mediates the relationship between menstruation and SNS use in adolescent girls. Results based on a large nationally representative sample in the United Kingdom suggest that the age of menarche was negatively associated with daily hours of SNS use, and that BMI elevation partially mediated this association. These results extend the negative effects of the early onset of menstruation and imply that BMI control strategies may help to reduce the use of SNS in girls who experience menarche at an earlier age.