Facebook posts compete for human attention in a zero-sum game; this makes it a challenge for government organizations to engage with their citizens through this medium. In a large-scale longitudinal study we investigate what makes Facebook posts popular (seen by many) and effective (commented, liked, or shared by many) in a nonprofit context: the official Facebook page of a midsized city (Oulu, Finland). We model the competition dynamics that shape the fate of Facebook posts using Structural Equation Modeling. Our analysis reveals that audience demographics, the timing of posts, and the media type of post are significant factors in post reach and effectiveness, and argue that our method can be applied to other contexts to determine which factors can lead to improved government-citizen communication and engagement. Finally, we argue for more actionable research to be conducted on the use of social media by government.