Recommendations to improve university Science, Technology, Engineering, and Mathematics (STEM) education often emphasize the role of classroom assessment in supporting student learning. Despite extensive efforts to support instructors in reforming their practices, many continue to teach didactically with limited classroom assessment. Instructors' decisions to adopt new practices have previously been characterized with the innovation decision model, and the spread of such practices can be explored through an innovation diffusion lens. Innovation diffusion of high-impact assessment practices requires interaction between experienced assessment users and less-experienced users. Within a university STEM department, instructors' assessment experiences were documented through the Faculty Self-Reported Assessment Survey (FRAS), and interactions between instructors were characterized with social network analysis. Results show that instructors with higher self-reported assessment experience had more teaching-specific peer interactions within the department, and that instructors of all assessment experience levels interacted more often with more experienced instructors. This study demonstrates and characterizes the conditions for innovation diffusion in a university STEM department and highlights the potential role of peer interactions for supporting the spread of innovative teaching ideas.