Being dominant factors driving the human actions, personalities can be excellent indicators in predicting the offline and online behavior of different individuals. However, because of the great expense and inevitable subjectivity in questionnaires and surveys, it is challenging for conventional studies to explore the connection between personality and behavior and gain insights in the context of large amount individuals. Considering the more and more important role of the online social media in daily communications, we argue that the footprint of massive individuals, like tweets in Weibo, can be the inspiring proxy to infer the personality and further understand its functions in shaping the online human behavior. In this study, a map from self-reports of personalities to online profiles of 293 active users in Weibo is established to train a competent machine learning model, which then successfully identifies over 7,000 users as extroverts or introverts. Systematical comparisons from perspectives of tempo-spatial patterns, online activities, emotion expressions and attitudes to virtual honor surprisingly disclose that the extrovert indeed behaves differently from the introvert in Weibo. Our findings provide solid evidence to justify the methodology of employing machine learning to objectively study personalities of massive individuals and shed lights on applications of probing personalities and corresponding behaviors solely through online profiles.The online social media has being becoming an essential component of everyday life, which even reflects all aspects of human behavior. Millions of users have digitalized and virtualized themselves in popular platforms like Twitter and Weibo, including basic demographics, plenty of statuses, abundant emotions and diverse activities. These online profiles can be natural, detailed, long-term and objective footprints of massive individuals and thus they could be promising proxies in understanding human personalities [1,2]. Since its beginning being a sub-discipline of psychology, the study of human personalities has aimed at one general goal, which is to describe and explain the significant psychological differences between individuals. Revealing the connection between different personalities and corresponding behavioral patterns, especially in the circumstance of online social media, is one of the most exciting issues [3,4,5] in recent decades. And a growing body of evidence implying individual personality discrepancy in online social media further makes it imperative in probing online human behavior from views of personalities [6,7,8].Personality is a stable set of characteristics and tendencies which specify similarities and differences in individuals' psychological behavior and it is also a dominant factor in shaping human thoughts, feelings and actions. However, personality traits, like many other psychological dimensions, are latent and hard to be measured directly. Self-report of asking subjects to fill survey questionnaires referring to personalities is a classical ...