Improving face-to-face (f2f) interaction in large classrooms is a challenging task as student participation can be hard to initiate. Thanks to the wide adoption of personal mobile devices, it is possible to blend digital and face-to-face interaction and integrate co-located social media applications in the classroom. To better understand how such applications can interweave digital and f2f interaction, we performed a detailed analysis of real-world use cases of a particular co-located social media app: SpeakUp. In a nutshell, SpeakUp allows the creation of temporary location-bound chat rooms that are accessible by nearby users who can post and rate messages anonymously. We find that the use of co-located social media is associated with an increase in content-related interaction in the class. Furthermore, it is associated with an increase in the perceived learning outcomes of students compared to a control group. We further provide design guidelines to blend digital and f2f interaction using co-located social media in the classroom based on 11 case studies covering over 2000, students.