This research investigates the interplay between urban leadership, social media, and sustainable development by analyzing the X activity of 23 European city mayors. Utilizing a comprehensive dataset of 48,000 X (formerly Twitter) posts, the study aims to understand how mayors utilize this platform to communicate with urban communities and advance sustainable development goals. The dataset encompasses posts collected from the profiles of 23 mayors of major European cities, spanning their entire posting history on the platform up until March 2023, and the analysis also includes an examination of their follower networks. The research explores the relationships between mayoral network centrality, research and development (R&D) investment, and urban sustainability performance. Findings reveal a strong correlation between R&D investment and mayoral network centrality, suggesting that cities with significant R&D investments tend to have more influential mayors on X. Furthermore, the analysis demonstrates a positive correlation between the frequency of SDG-linked posts and urban sustainability performance, highlighting the possibility that social media posting can serve as a good proxy for advancing or communicating sustainable development agendas. Network analysis reveals the importance of collaboration and knowledge sharing among cities, with central nodes playing a pivotal role in facilitating information exchange and driving innovation.