In order to reap the social gratifications of Online Social Networks (OSNs), users often disclose self-related information, making them potentially vulnerable to their online audiences. We give a brief overview of our theoretical ideas and empirical research about additional cognitive and metacognitive factors relevant for the perception of risk when self-disclosing information in OSNs. More specifically, we discuss disclosure related "knowledge" and "metacognitive accuracy" and describe three studies in which we investigated if OSN users knew what contents they had disclosed, to which audiences these were accessible, and if users were aware of the extent of their own knowledge (metacognitive accuracy). We found that OSN users remembered their disclosed contents well but struggled to remember specific audiences. Additionally, they were hardly aware of these knowledge differences.