Social networking sites (SNS) are helpful for stirring up interactions among users. The number of libraries which adopt SNSs is increasing. However, user engagement is low on many libraries' SNSs. Existing research mainly focuses on the ways SNSs used in libraries and the librarians or users' attitudes towards libraries using SNSs. Little research has been done on how to use SNSs to interact with library users effectively. This study focuses on the interactions between libraries and users on libraries' Facebook, Twitter and Weibo. Four types of interactions are examined, including knowledge sharing, information dissemination, communication and knowledge gathering. A mixed method is applied in this study: quantitative results, generated from the analysis on around 1700 posts sampled from 40 libraries' SNSs, are incorporated with qualitative results concluded from the interviews with 10 librarians. The study finds that among the four types of interactions, knowledge sharing attracts the largest volume of user responses on libraries' SNSs. The study's investigation on the differences of Facebook-like and Twitter-like SNSs and those between academic and public libraries on using SNSs suggests that in order to improve the efficiency of interacting with users on SNSs, there are necessities for libraries to coordinate different types of SNSs and take the properties of their communities under consideration.