Colleges share many things on Twitter, but one topic can be risky to broach: the reading habits of library patrons. Patrons' privacy is precious to most librarians. Yet new Web services thrive on collecting and sharing the very information that has long been protected. This points to an emerging tension as libraries embrace digital services. Historically, libraries have been staunch defenders of patrons' privacy. Yet to embrace many aspects of the modern Internet, which has grown more social and personalized, libraries will need to "tap into and encourage increased flows of personal information from their patrons," says the privacy-and-social-media scholar Michael Zimmer. But as librarians expand digital services, they face "a Faustian bargain," warns Mr. Zimmer, an assistant professor in the School of Information Studies at the University of Wisconsin at Milwaukee. In a forthcoming paper, he writes that librarians may decide that "the benefits of these advanced data-based services outweigh the traditional protection of patron privacy." Now the Web has put privacy in flux, and the lines are fuzzy as to what trade-offs libraries should make. When should data be used? When should the information be shielded? One option is to use systems that allow patrons to opt in to libraries' tracking such activities as their previous checkouts.