The sustainability of social networking sites is critically dependent on information sharing by site users. However practice suggests that privacy is a major concern which affects users' willingness to share personal information on social networking sites. To understand how privacy expectancies affect sustainability we draw on the psychological contract theory. We develop a framework which posits that privacy-related expectancies are inherent to disclosure of private information. Perceptions of a privacy breach trigger the affective experience of a psychological contract violation and undermine trust, which consequently undermine the sustainability of continued information disclosure. We evaluate the framework by surveying 638 Facebook users. We find that SNS users' perceptions of privacy breach attributed to the SNS provider may cause irreparable harm. The experience of psychological contract violations and loss of trust motivate users to reduce information sharing and consider disengaging from the SNS completely.