Individuals high in neuroticism experience negative affect and social anxiety, therefore, they may prefer online communications where they are able to portray an idealized image of themselves to attract social support, seek validation, and enhance mood. These motivations may lead to greater social media use and addiction. In this study of Facebook users (N = 742; 474 females, 268 males) who mostly resided in the United States, we explored the association between neuroticism, Facebook addiction, and negative affect. Our mediation model showed a partial mediating effect of Facebook addiction in predicting negative affect in individuals who were high in neuroticism. In theory, individuals who use Facebook as a tool to improve their mood may actually experience more negative affect due to social overload, jealousy, and envy. Individuals high in neuroticism, who already maintain a higher negative affect than those low in neuroticism, might be particularly inclined towards increased Facebook use, which could deteriorate their mood further. Consequently, individuals and therapists should be aware of the personality traits that could lead to greater Facebook addiction and a higher negative mood. Therapists could encourage users to engage in activities, other than social media use, that can improve mood without leading to addiction.