This paper aims to present a study on the appropriation of the social media Facebook by four Brazilian State Public Libraries: the Public Library of the State of Acre, the Public Library of the State of Parana, the Public Library of the State of Pernambuco and the Public Library of the State of Sao Paulo. The study, based on literature review, addresses the concepts of Public Libraries, sociotechnical mediation and social media. It questions the role of the Public Library in the contemporary society and claims that libraries should, through mediation, recognize the importance of their space as a room for dialog, exploiting it as a means of communication and interaction. The study also analyses the messages posted in the aforementioned Libraries in the period that goes from January to June 2013. It concludes that the social media studied is not utilized at its full potential.