Emotion entrainment refers to the phenomenon that people gradually synchronize to other's emotion states through social interactions. Previous studies mainly focus on conducting laboratory experiments or small-scale offline surveys. Large-scale empirical studies on real-world emotion entrainment among individuals are still to be explored. Especially, determinants that influence this process are not clear. Also, how emotion evolves among people in a large scale population is still unknown. In this study, we attempt to conduct a large-scale empirical analysis on emotion entrainment based on online social media information. For this purpose, we develop a model-free framework to measure entrainment strength among people. Experimental results indicate that interaction partners with strong reciprocal entrainment tend to assume similar emotion states, and negative emotion is more empathetic in an intimate relationship. Especially, when the relationship is balanced, users are more emotionally similar to each other.