Definition: Viral marketing refers to a technique in marketing a product or a service where users help in spreading the advertiser’s message to other websites or the users create a scenario which can lead to multi-fold growth. In other words, what would previously have been the entire audience for the message also becomes the big seed for a viral campaign in which the newly added people can forward the message to their friends, who may forward it to their friends in turn, and so on.
Definition: Viral Marketing (viral advertising, or marketing buzz) marketing technique that use pre-existing social networking services and other technologies to try to produce increases in brand awareness or to achieve other marketing objectives (such as product sales) through self-replicating viral processes.
Her research into social media marketing, content marketing and video sharing have been internationally recognised both in industry and academic forums while her (sometimes controversial) findings regularly spark global discussion amongst practitioners.
Again, I stress that this is a legitimate marketing tactic which can achieve Fantastic results if it is done correctly, and as Internet marketing is extremely competitive you should use all the avenues that are available to you in advertising your web site and products.
In your Excel example (row 25), with 10 initial users in time period 0 to 10 with a cycle time of 10 and a viral coefficient of 2, you show 10 users growing to 30. That makes sense to me because 10 users each brought on 2 additional users, thus 30 total with the original 10. In the next period though you show 70 users.