Blue spaces, water-based open spaces, are becoming focal points for urban vitalization. While previous studies have explored waterfronts’ various effects, little research has focused on their influence on actual visitation and vitality. This study addresses this gap by analyzing the effect of riverfronts on mobility using de facto population data, which tracks citizen activity by location through mobile information. The study focuses on two major rivers in northwestern Seoul, covering nine major riverfront facilities. Population Vitality (PV) and Vitality Index (VI), two novel measures derived from the de facto population data, were calculated for 266 tracts and used as a new indicator of spatial activation. Explanatory variables include regional and riverfront factors, such as the density of facilities, riverfront spaces’ specifications, and vitalization patterns. The findings show that higher densities of park-green spaces and commercial activities significantly enhance vitality, aligning with previous research on open spaces. Compact riversides with higher densities of riverfront facilities also exhibit greater vitality. The VI has demonstrated feasibility as a dynamic metric for assessing spatial activation, effectively capturing temporal fluctuations. By utilizing population big data and novel indices, this study empirically demonstrates the magnetic effects of riverfronts, providing deeper insights into effective riverfront planning.