In this paper we present our experience in building a socio-technical infrastructure for supporting social innovation in Information Technology. We start by describing a case study on the design and use of a smartphone application for the canteen services of a local university; based on this, we propose what we call the hourglass approach to support participatory design and development in Information Technology. The hourglass is defined by the intersection of two co-evolving dimensions of infrastructuring: the social and the technical ones. Different subsets of the community, characterized by the increasing involvement of self-selected volunteers, position themselves along the two axes and have different roles in the design and use of the generated artefact. We conclude by discussing how this approach can help addressing some of the current challenges (i.e. scale, milieu and responsibilities) of social innovation in Information Technology.