Facebook remains the most widely used social media platform. Research suggests that Facebook may both enhance and undermine psychosocial constructs related to well-being, and that it may impair physiological stress recovery. However, little is known about its influence on stress reactivity. Using novel experimental methods, this study examined how Facebook influences reactivity to an acute social stressor. Facebook users (n = 104, 53 males, mean age 19.50, SD = 1.73) were randomly assigned to use their own Facebook account or sit quietly with the option of reading electronic magazines before experiencing an acute social stressor. All participants showed significant changes in subjective and physiological stress markers in response to the stressor. However, participants who used Facebook experienced lower levels of psychosocial stress, physiological stress, and rated the stressor as less threatening (p's < 0.05) when controlling for gender and emotional investment in the website compared to controls. Results suggest that Facebook use may buffer stress—in particular psychosocial stress—if used before experiencing an acute social stressor. This study is among the first to incorporate both objective and subjective measures in investigating the complex relationship between Facebook use and well-being.