Placeness, the combination of physical, social, and cultural meanings of a place, is used in explaining and finding urban characteristics of public places. The concept of placeness has long been studied in various areas like computer science as well as social studies and architecture, for its rising importance to urban planners and engineers as it successfully provides the knowledge base of locations. Here, we explore placeness through Instagram, a well known social media, as it works as a public diary for all users, providing rich information about their daily events in real time. We focus on two urban regions in Seoul, Gangbuk and Gangnam. We conduct our research based on a ontology-based placeness model which can discover polymorphic characteristics of urban areas. Next, we collect Instagram posts as a dataset from Gangbuk (hotspots near 14 subway stations of Gangbuk area) and Gangnam (hotspots near 11 subway stations of Gangnam area) and transform the data into a computable format using machine learning techniques. Finally, we identify statistical differences between the two regions following known social studies, as well as conduct regional analysis on their 25 districts. Our results show that there is a social gap between Gangbuk and Gangnam, which is visualized from Instagram posts, known as binary opposition structure.