Of key importance to avoiding significant health problems such as cardiac disease and stroke is eating nutritious foods and leading an active lifestyle. However, leading a healthy lifestyle remains elusive and obesity continues to increase in North America. We investigate how online social networks (OSN) can change health behaviour by blending theories from health behaviour and participation in OSNs. We designed, prototyped and evaluated an OSN, called VivoSpace, using factors derived from the blend of theoretical models with the intention to change health behaviour. Our results from field studies with 35 participants from two clinical and one non-clinical social network groups revealed that designing these factors into the online social network lead to positive health behaviour change, namely, improved diet for the clinical group. The use of theoretical models in the evaluation provided a means to understand how the design can be improved to see further health behaviour change.