Social media offer governments a new approach to create transparency and accountability, to increase citizens' opportunities to participate and collaborate in decision making or to enable the improvement of public services. This study is a first step toward understanding the use of social media tools by local governments for transparency purposes. The aim of this paper is to construct a synthetic indicator to measure Facebook use by local governments to disclose information and analyse the determinants of varying levels of Facebook use. Our sample consists of 217 Italian and Spanish local governments which used Facebook in 2014. Our findings show that level of e-participation, population size, citizens' income level and level of indebtedness have an impact on Facebook use by local governments. A high level of Facebook use would suggest that local governments tend to enable citizens to oversee government making more transparent information, data and processes. Thus, this study contributes to the debate on local government transparency and provides important guidelines for developing appropriate social media strategies and policies.