Masspersonal communication on social media is disrupting the boundary between interpersonal and mass communication. The purposes of this study were to examine emerging adults’ norms for masspersonal communication on social media, and to test whether privacy concerns, narcissism, and self-monitoring drive perceptions of those norms. Vignettes were presented to 663 U.S. college students to measure the perceived appropriateness of seven kinds of masspersonal self-expression (emotions, political views, social events, romantic affection, professional achievements, location, and sexy photos). Participants also were assessed for privacy concerns, narcissistic personality, and self-monitoring. Professional achievement was considered the most appropriate topic for masspersonal communication on social media. Political views, romantic affection, and social events also were considered appropriate. Emotions, location, and sexy photos were not. Narcissism and privacy concerns predicted endorsement of self-expression in the directions hypothesized. Self-monitoring was associated with endorsement of self-expression only regarding the topics found appropriate by the overall sample, and endorsement of self-censorship in the inappropriate ones. These findings indicate that shared norms exist regarding masspersonal communication, that they inform emerging adults’ behavior, and that individual motivations (related to narcissism, privacy concerns, and self-monitoring), influence how those norms are understood.