The aim of this study was to explore how Japanese young adults use social networking sites (SNS) and the social psychological factors that might impact their SNS security settings. A survey (N = 79) was employed to examine usage habits, actual account settings, risk perception and social cognition related to Twitter. Results indicate that there are discrepancies among privacy concerns, usage habits and actual privacy settings in Twitter users. More than 80% of respondents actually use public settings on their accounts, and a half of them disclose personal details in their profile despite believing that private information should not be disclosed on Twitter. Respondents also exhibited optimistic cognition and behavior regarding online privacy risks for themselves and negative stereotypes toward Twitter users whose accounts are protected. The possibility that these social cognitive factors have an impact on user privacy settings is discussed in this paper.