An emerging trend in classroom technology research is the use of computer mediated communication (CMC) tools in classrooms to encourage students' in-class participation. As part of this research thread, we have been investigating the potential of public digital backchannels for building feelings of community among students in university courses. We designed, deployed and evaluated such a tool in a 15-week field study of two undergraduate classes. We found students found using public backchannel during the class is of little distraction, that teachers' attention to the content posted on the channel influence students' tendency to use tools of this kind. Further, we found that the relevance of the content shared is predictive of students' use of ClasCommons in the classroom; these feelings in turn are related to students' perceptions of selfefficacy, collective efficacy and course-specific social support. We also analyzed the content posted in the public backchannel and considered the benefits and drawbacks of the public digital from both students' and teachers' perspectives. We conclude with suggestions for improving the design and deployment of courserelated backchannels.