Viral Marketing on Facebook

In brief: It is not viral if no one wants to share it. The good thing happens when the right people forward the message to the right people.

Let's imagine that you have started a group dedicated to the art of eating with chopsticks (or the benefits of drinking beer). You invited your hundred friends and twenty joined the group. All you guys started sharing ideas and stories related to eating with chopsticks. Some really useful and entertaining dialogs happen.
Out of the twenty people that you invited ten decided to spread the news and send invitation to their circle of friends. If we assume that everyone has hundred friends, which automatically mean that the news for your newly created group has reached thousand readers.
Although a bit optimistic, this is the way how things can happen. I have noticed that interesting groups that have 1000-2000 members grow with average 10 members per day, while bigger groups grow with hundreds, even thousands members per day.