This paper examines the role of the enterprise social networking (ESN) tool Slack in the daily work of software practitioners within PubComp, a large-scale agile public sector organization. Based on 13 interviews with PubComp's developers, our case study explores how Slack is employed for knowledge sharing and daily communication across the organization. We used a newly developed framework for communication in agile teams as a theoretical lens. Through our analysis, we found that Slack use had become deeply integrated into the organizational culture and fostered alignment in three main ways: Promoting \textit{communication transparency} through open discussions visible for PubComp developers organization-wide, enhancing \textit{communication quality} with prompt responses and constant communication, and encouraging \textit{communication discipline} through structured channels and threads. These categories also came with their own sets of challenges, however, that we suggest can be overcome by employing three straightforward yet potentially practical organizational ESN user guidelines. Yet, our findings suggest that if common hurdles are overcome, modern ESN tools can reshape how developers in the public sector communicate, reinforcing the agile principles of shared responsibility and collaboration.