This article analyses Social Media (SM) use as a promotion tool for public institutions in the public audit sector. The authors propose a quantitative model to assess online engagement of 94 European audit institutions (national and regional) with their stakeholders, based on SM and web activity metrics of these institutions, with a focus on pressing matters such as environment, sustainability and the current COVID pandemic. The proposed model may be applied to assess organisations from any public or private sector. The research finds that SM presence helps audit institutions to promote their services more effectively by directing their stakeholders to extensive content on the audit institutions’ websites, thus contributing to an increase in their web traffic. Most audit institutions do not have a strategy for the online promotion of their services and work outputs, nor do they, in general, use SM for intensively promoting environmental or sustainability-related messages. The only exception is the European Court of Auditors, who clearly promotes its environment-related activity by actively and increasingly posting on SM and referencing its reports. Audit institutions could benefit from the nature of their evidence-based activity and promote their findings on emerging topics to positively influence public policies and accountability at all levels.