This paper reports on a study that explored the user-driven inter-organisational professional learning that teachers from two education systems collectively engaged in via social media. A dedicated Facebook group was established to enable Australian early childhood teachers (sector one) and primary school teachers (sector two) to engage in collaborations that would support children's transition to school. Using an Activity Theory framework, findings showed that the site was mainly used by early childhood teachers to seek peer support in meeting reporting requirements, and that school teachers rarely posted. As such, the capacity of the Facebook group to support inter-organisational cross-sector collaboration and learning was challenged as determined by the most active participants within the site, while reinforcing the group's capacity to support 'just in time' intra-organisational professional learning. This highlights a need for further research to explore the ways and means by which social media may best facilitate cross-sector collaboration between education systems, such as a more focussed and integrated use of social media during face-to-face cross-sector professional learning.