The study empirically investigated the extent of utilization of social media for effective Library and Information Service delivery in selected university libraries in Abia State, Nigeria. five research questions guided the study. The study adopted descriptive survey research design. Using multi-stage sampling technique 350 library users and staff were sampled from the entire population. Questionnaire and Observation Checklist was the instrument for data collection constructed for the study. The data collected was analyzed using frequency distribution table, charts, percentage and mean. The findings revealed that, both ABSU, Library and MOUAU, Library has no official facebook, twitter, Instagram, WhatsApp, Nairaland, Skype, Googletalk, Academia.edu, researchgate, Mendeley, Ning, Librarything, Flickr among other social media sites. The findings revealed that due to the lack of official social media sites in both university libraries studies, social networking sites are not used in providing Library and Information services. The respodets perceived social media as a great media for effective library service delivery. Obsolete Internet facilities, Technophobia, Poor communication systems, Lack of social networking sites use, Lack of training of staff in the use of computer, Inadequate computerization, Lack of awareness of social networking sites, insufficient bandwidth, Lack of qualified ICT librarians, Erratic power supply was identified as the challenges associated with the utilization of social media for effective Library and Information Service delivery in selected university libraries in Abia State, Nigeria. the study recommended that, Social Networking Site use Policy should be formulated in the Library, Younger Librarians who are digital natives should be in charge of using social networking sites for providing Library and Information Services to Library users among others.