The impact of state restriction and surveillance on social media engagement has been widely investigated in communication studies. However, these studies tend not to capture the moment when these restrictions are implemented and citizens experience a high level of uncertainty. Addressing the implementation of a national security law (NSL) in Hong Kong, we used two-wave panel data to understand political engagement on Facebook before and shortly after the law was enacted. We find a serial mediation path in which pan-democratic and localist users (those who tend to oppose the government) showed greater fear of the law; this encouraged more active privacy management which was related to a higher level of engagement after the enactment of NSL, implying that these users responded strategically through restricting the visibility of their engagement. This mediation path is moderated by the level of political disagreement users encountered on Facebook. Implications of the findings are discussed.