Cross-platform government social network studies have focused on the messages that are being sent, and less on community responses the content generates. Moreover, little attention has been paid to possible contextual drivers of online citizen engagement. This article assesses 505 social media posts made by a sample of Dutch municipalities across four different platforms (Twitter, Facebook, LinkedIn, Instagram), their policy area, and the level of engagement they have fostered. The findings support the proposition of policy discretion as contextual driver of online citizen engagement: when municipalities can exert more discretionary power in a policy area, citizens could feel empowered to engage, as they could personally persuade decision makers. Moreover, although Facebook fostered the highest levels of engagement, the platforms show a multifaceted picture with respect to engagement levels and policy area. This suggests that local governments should treat platforms as distinctive realms, requiring a holistic approach to online citizen engagement.