Social network sites, such as Facebook, allow users to create novel representations of the self that capture both social connectivity and important personal attributes (e.g., social roles, activities, hobbies and values). According to self-affirmation theory, this type of information could be self-affirming in the sense of reinforcing an overall sense of self-worth and emotional well-being. The theory posits that once affirmed by attending to such personally meaningful information, individuals tend to be more secure and less defensive when confronted with the slights and challenges of daily life. This dissertation tested the hypothesis that Facebook profiles constitute an everyday source of self-affirmation, as understood through the lens of self-affirmation theory. Study 1 found that a brief exposure to one's own Facebook profile had the same psychological effects as a classic self-affirmation manipulation, namely reduced defensiveness when confronted with a threat to the ego (i.e., negative feedback on an academic task). Facebook profile exposure also resulted in increased positive affect, both self-directed (e.g., feeling loved, supported and connected) and other-directed (e.g., feeling loving, giving and grateful). Study 2 examined whether Facebook users take advantage of these self-affirmational benefits in times of psychological need. As predicted by self-affirmation theory, participants were more likely to choose to spend time on Facebook when their ego was threatened than when it was unharmed, suggesting that Facebook use is partly motivated by an unconscious need to restore self-worth. Study 3 examined further perceptual and behavioral consequences of Facebook profile exposure and found that, while Facebook raises state self-esteem, it can harm performance on a subsequent task by reducing motivation to expend effort on it. Together, these studies contribute to self-affirmation theory by examining how self-affirmation operates in everyday life, outside of experimenter-generated selfaffirmation interventions. Results also illuminate previously unexplored motives for Facebook use (i.e., an unconscious desire to elevate feelings of self-worth), and highlight psychological benefits (i.e., increased positive affect and state self-esteem, and decreased defensiveness) and costs (i.e., reduced motivation for task performance) resulting from Facebook profile exposure.