In this study, we investigated personality and mental health variables for users of three popular social networking services (SNS): Twitter, Instagram, and Facebook, among Japanese undergraduate students (N = 1030). Compared to non-users, Twitter users scored significantly lower in extraversion, conscientiousness, openness, and self-esteem, and higher in neuroticism, behavioral inhibition system (BIS) sensitivity, and social comparison orientation. Instagram users scored significantly higher in extraversion, behavioral approach system (BAS) -drive, BAS-reward responsiveness, BAS-fun seeking, narcissism, self-esteem, and subjective happiness, and lower in neuroticism, anxiety, and to a marginal extent, depression. Facebook users scored significantly higher in extraversion, openness, BAS-fun seeking, self-esteem, and to a marginal extent, narcissism. Within SNS users, extraversion, agreeableness, and self-esteem were negatively associated with negative mental health variables, and positively associated with positive mental health variables. Neuroticism was positively associated with negative mental health variables, and negatively associated with positive mental health variables. Further, some associations were found within specific SNS users. Differences in personality and mental health were discussed based on the characteristics of each SNS platform.