At the risk of stating the obvious, successful viral marketing is one whose average forwarding rate exceeds 1, i.e. in which on average, every person who uses the product or service gets more than 1 other person to use it. This can be achieved by getting all users to tell 1 or more friends, or more frequently, by getting at least 1/Nth of the users to tell N or more people. In other words, a small base of passionate fans can do just as much as a large base of mildly pleased users.