Informal learning in blended and online settings plays an increasingly important role in the continuous professional development of individuals. Yet, how do individuals engage into these types of activities? We argue that social capital theory can provide valuable insights into how people behave and decide to take part in (in)formal learning. Using social network analysis, we provide empirical evidence on an informal learning activity, where participants collaboratively created knowledge on wiki sites on a predefined set of topics. Our results show that there are three types of active participants in these types of informal learning activities, namely "leaders", "collaborators" and "lone editors". These findings provide valuable insights for other, similar activities that aim at enhancing the professional development of individuals.