To scale up pedagogical innovation using information and communications technology (ICT), educators have shifted the focus of professional development from primarily an individualist endeavor to a collective approach including teachers' social networks (SNs). SNs have been found to be a major contributor to professional development and support, which in turn promote teachers' self-efficacy and improve their teaching practices. However, the effects of SNs in pedagogical innovation may vary depending on the structure of SNs and the quality of the interaction in a network. It is necessary to reconsider the assumption that the more networked teachers become, the better they teach students. This study aims to explore both the supportive and obstructive roles of teachers' SNs in scaling up ICT-based instruction through in-depth interviews with 14 primary school teachers (7 active and 7 passive ICT using teachers) in South Korea. This study found that some SNs played a positive role in supporting ICT-based instruction and professional development, whereas some imposed constraints on using ICT for teaching and learning. Teachers who actively conducted ICT-based instruction were likely to have appropriate countermeasures to obstructive SNs. This study suggests that systematic supports are necessary to help teachers expand their supportive SNs in and out of schools and to develop professional competencies to overcome the negative impacts of obstructive SNs.