What do academic libraries tweet about, and what makes library tweet useful?

People spend an increasing amount of time using social media systems to network, share information, learn, or engage in leisure activities (e.g., gaming). Libraries too are establishing a social media presence to promote the library and provide services to user populations through the social media systems the users frequent. This study explores Twitter uses by six large academic libraries, and factors that make library tweets useful. 752 tweets were analyzed by topic to develop a subject typology of library tweets. In addition, tweets and Twitter user characteristics were analyzed to explore what makes library tweets useful, as measured by the number of retweets and favorites received. Content analysis of the samples of library tweets revealed nine content types, with the Event and Resource categories being the most frequent. In addition, the analysis showed that tweets related to study support services and building and maintaining connections with the library community were the most frequently retweeted and selected as favorites. The presence of a URL in the tweet was positively associated with the number of retweets, and the number of users followed was positively associated with the number of favorites received. Finally, a negative correlation was found between the account age and number of favorites.