Purpose. The purpose of this paper is to investigate the presence of academic libraries at the Josip Juraj Strossmayer University in Osijek on the social network Facebook, and to analyze their activity. A quantitative and qualitative analysis of the seven academic libraries’ posts that have their own Facebook pages was carried out to determine which content on the sites is most popular and how much Facebook can help libraries to promote their activities and establish better communication with users. Approach/methodology/design. By browsing the websites of all University components that have libraries, it was found that 7 out of 15 libraries have their own Facebook pages. By determining the number of „likes“ and the number of followers, the popularity of each library Facebook page was estimated. In the next step, the activity of librarians on the Facebook pages was observed by counting published photos, videos, events, and notes. The data on the posts published by the libraries and the reactions of the followers were analyzed in detail quantitatively and qualitatively for the period from January 1st to April 30th 2020. Finally, the most popular posts on library Facebook pages were highlighted. Results. The analysis showed that less than half of the libraries of the Josip Juraj Strossmayer University in Osijek do not have their own Facebook page. The Faculty of Humanities and Social Sciences has the largest numbers of „likes“ and followers on their page. We have not been able to prove the assumption that the numbers of „likes“ and followers of the pages are influenced by the size of the total number of students of a certain faculty, the date of creation of the site, and whether the faculties belong to the social sciences or STEM. The popularity of the Facebook pages, however, depends mostly on the type and content of the posts, the number of photos, videos, and events. The best indicator of popularity is the number of reactions to the posts, the number of comments and shares. According to these indicators, we found that the Facebook page of the library of the Faculty of Humanities and Social Sciences is the most popular library Facebook page at the University. Originality/value. The paper brings previously unpublished data on the presence of academic libraries of the University of Osijek on the social network Facebook, the quantitative and qualitative data on their activity, and the overall user acceptance of the posts written by the libraries. The detailed content analysis of the libraries’ posts on their Facebook pages can serve as a guide for libraries in strategic planning and identifying the needs of their users, primarily students.