Personal networks are complex variables characterizing the social resources and constraints of individuals and can be used to explain outcome such as behaviour, health, or job performance. Collecting qualitatively rich personal network data, however, is costly and difficulties increase if longitudinal data is needed. In this paper we identify requirements that collected personal network data should satisfy to be suitable for longitudinal analysis. Likewise we identify requirements on the tools for collecting personal networks and study which of these are satisfied by current data collection tools. We present design aspects and a prototypical implementation of a smartphone app (dubbed the Social Capital Companion) allowing users to keep a personal network diary on their own mobile devices over potentially long periods of time.