Abstract: Users of social networking sites (SNSs) increasingly must learn to negotiate privacy online with multiple service providers. Facebook’s third-party applications (apps) add an additional layer of complexity and confusion for users seeking to understand and manage their privacy. We conducted a novel exploratory survey (conducted on Facebook as a Platform app) to measure how Facebook app users interact with apps, what they understand about how apps access and exchange their profile information, and how these factors relate to their privacy concerns. In our analysis, we paid special attention to our most knowledgeable respondents: given their expertise, would they differ in behaviors or attitudes from less knowledgeable respondents? We found that misunderstandings and confusion abound about how apps function and how they manage profile data. Against our expectations, knowledge or behavior weren’t consistent predictors of privacy concerns with third-party apps or on SNSs in general. Instead, whether or not the respondent experienced an adverse privacy event on a social networking site was a reliable predictor of privacy attitudes.