‘Desktop’ social networking services are migrating to mobile devices. Research into the design of mobile social software (MoSoSo), especially its communication design, is emerging. The case we present is from a collaborative, interdisciplinary research project into communicative design innovation concerning these technologies. In focus is the design of what we label the communicative prototype for an interaction and media centred view of social software development. This view is applied to an exploratory design research case that extends an established online social service to the iPhone/iPod platform. The conceptual design in the case is intended to enable the discovery of independent, non-commercial music. The projected service was developed in consultation with a national public service broadcaster. We frame the design and analysis within a sociocultural approach to mediated communication and research by design. We employ mixed methods both in design and in research. We argue that a communicative stance in early concept development offers valuable insights on the ongoing design of social software. The communication expertise of interaction designers is central to this.