Viral marketing is a form of "online word-of-mouth" or more precisely "a marketing technique developed on the Internet and designed to promote the circulation of a message to sites or users in order to exponentially increase its visibility and effectiveness,” according to Guillaume Bernard and Frédéric Jallat.
For your information, we differentiate between active viral marketing where internet users recommend a job offer to a close friend, of passive viral marketing where internet users participate in the promotion of the service or brand by their simple status as a user (example: email signature from the Yahoo! online service).
