Recent literature has suggested that smartphone addiction is negatively associated with users' psychosocial well-being. Much of the research on this subject, however, is of a correlational nature, which has been pointed out as an important limitation that does not allow distinguishing the antecedents of the consequences. In this study, 416 smartphone users were followed for 1 year (three waves separated by 6 months each) to assess the relationship between smartphone addiction and social support. Cross-lagged model results indicated that social support predicts later addiction to the smartphone and that smartphone addiction decreases social support over time. Growth mixture model results indicated that the decrease in social support during the follow-up year was higher for users with greater smartphone addiction at the beginning of the study. Multivariate and univariate analyses of variance indicated that some personal characteristics of users (extroversion, neuroticism, and sensation-seeking) could affect the evolution of social support related to smartphone addiction. In general, these results suggest that the extensive use of a social communication technology such as the smartphone could have the paradoxical effect of diminishing the psychosocial well-being of its users.