Twitter is a popular social media tool for people to connect and communicate with one another. In recent years, digital libraries (DLs) have added Twitter to their lists of social media tools as a way to engage with users. However, how digital librarians and users actually use this social media tool is unknown. In order to gain more knowledge about the use of Twitter, the authors conducted an open coding analysis of Tweets from 15 different DLs for the course of one year. Five types of functions of Twitters in DLs were selected and discussed, which represent the problems, promotion, related resources, social connection and social identity of DLs. In addition, their relationships were also explored.