Objectives: Social media usage is increasing across the general population, and can lead to anxiety, depression, poor self-esteem, and poor body image (Kelly et al., 2018). The aim of the present study was to examine the effects of message framing (prescriptive and proscriptive) and self-affirmation manipulations on social media usage, psychological reactance towards messages, and cognitions towards reducing the use of social media usage. Method: Participants (N = 105) completed two online questionnaires. They were first asked to complete a value affirmation task (self-affirmation vs. no affirmation), and then to read a message about the negative effects of social media usage (prescriptive vs. proscriptive). They then completed measures of psychological reactance, intentions, attitudes, social norms, and moral norms surrounding reducing social media usage. Subsequent weekly social media usage and social media usage per session were measured 7 days later. The study employed a 2 (message frame; prescriptive vs. proscriptive) X 2 (affirmation: self-affirmation vs. no affirmation) between-subjects experimental design. Results: Seven ANCOVAs were used. The findings showed self-affirmation resulted in lower intentions to reduce social media usage than did no affirmation. Participants exposed to the proscriptive message reported spending marginally significantly less time on social media per session at follow up than those exposed to the prescriptive message. There were no significant effects on weekly social media usage, nor on psychological reactance, attitudes, moral norms, and social norms towards reducing social media usage. Conclusions: The study has implications for the future campaigns of public health organisations to encourage the general population to cut down on their social media usage in order to benefit their mental health. The strengths and limitations of the study are discussed, and directions for future research are suggested.