1. A method for determining a topical on-line influence of a commentor, comprising steps of: (a) matching and tagging content with a selected topic to generate tagged content; (b) extracting viral properties for the tagged content; (c) identifying a commentor from said tagged content; and (d) determining the topical on-line influence of said commentor; wherein said topical on-line influence of said commentor is characterized by a topical influence value of the commentor; and said topical influence value of the commentor is calculated from a commentor influence model based on the extracted viral properties.