<p>Social media has dramatically revolutionised the way people communicate and interact in the 21st century. The benefits of these tools are manifested in the increasing uptake worldwide by individuals, groups and organisations for knowledge exchange and marketing purposes. The academic/university library is no exception to this. As librarians seek to understand the ever-changing information needs of their customers, they must consider alternative means of interaction which social media offers. However, despite the attractiveness of social media outlets, university libraries cannot yet claim to have understood fully how to utilise them for marketing purposes effectively. Although studies on social media have received extensive attention in academic literature, little research has been conducted in the specific area of social media engagement. With many librarians bemoaning the lack of engagement from users on social media owing to negative attitudes, it is relevant to explore factors that affect sustainable social media engagement. This is a perspective that has been underexplored, particularly through the application of a strong theoretical base anchored on persuasion and attitude change. Underpinned by the theoretical foundations of the Elaboration Likelihood Model and the Strategic Social Media Marketing Framework, this study explores the factors that affect social media engagement of undergraduate student users with the university library. Employing a mixed-method approach, the study utilised semi-structured interviews, a questionnaire and content analysis to gather data from six university libraries in Nigeria that was used to examine the social media engagement phenomenon. The interview results revealed that participating libraries failed to plan the adoption and management of social media carefully. Evidently, among the participating libraries, only a few commenced with a defined purpose, which often is neither management-driven nor guided by policy. These purposes were focused on promoting library resources and services and getting traffic to the library website. Results of analysed social media data confirmed this, revealing that posts were made infrequently, lacked creativity, and generated a low engagement rate. The result of a multiple regression revealed that argument feature (post content, language, and type) is a significant influencer in the predictions of factors that motivate undergraduate students to engage with the library on social media, while an ANOVA test indicated that course offerings influence students’ attitude to the library and how they perceive the library on social media. The implications of these results are discussed, informing the theoretical and practical contribution. A principal theoretical contribution is a framework titled Sustainable Library Social Media Marketing Management that explains high-level social media management in the library. The study provides a blueprint for practising librarians with insights on managerial factors and considerations for user engagement as well as ideas for the purposeful planning of social media marketing activities.</p>