Viral marketing, known also as word-of-mouth marketing, refers to a marketing mode attached to some carriers on the Internet, which can rapidly copy marketing information at a low cost [1–4]. Different from the traditional marketing methods, for example by advertisement, viral marketing spreads the marketing information through word-of-mouth among the users, which turns out to be more trustful for many consumers. Noticeably, viral marking is different from the traditional word-of-mouth marketing, and it carries some advantages over the traditional method for it is launched via web-based communication platforms such as BBS, Web Blogs, instant messaging systems, and so on [5,6]. The main reasons are multi- fold. First, the information can be easily stored in computers and the web; therefore digital word-of-mouth marketing information exists everywhere on the Internet and for a long time. Second, most people can take part in the word-of- mouth marketing process in their leisure time, so by nature it is a non-synchronous spreading mode of advertisement. Third, the anonymity feature enormously reduces the limitation induced by personal identities thereby making mutual communications on the web much easier than face-to-face in real life. Last but not least, digital word-of-mouth marketing through the Internet can spread more widely and much faster at a significantly lower cost as compared to most if not all traditional methods [7].