You often hear people say that you should make your product inherently viral. An fully viral product is a product like Skype. If only one person uses Skype, then Skype has no utility at all. A non-viral product is something like a Tax Calculating software. You basically use this all alone and by yourself.
Fully viral products are dangerous because they have the chicken and egg problem. If only one person uses it, it’s useless, so when people try it, they abandon it almost immediately. To make it take off, you need to have a mass of people who use it all at once, and for that, you need a strong and sustained external user source. If you don’t have a user source, then making such a product take off is a very difficult marketing problem.
On the other hand, non-viral products require a lot of sustained expenditure on marketing. If you cannot afford to market, then should you really build such a product? If your product is going to be free, but it’s non-viral, then it becomes very hard for people to discover your product.
A solution is to make your products half-viral. That means that your product should have utility when used alone, but has some functionality that only works when shared with others, and that enhances the product a lot. A prime example is dropbox: It works good as a backup tool when you use it on your own. When you use it to share files, it becomes a viral tool. Half viral tools do not have the chicken-and-egg problem, and they do not need sustained marketing, because once enough people are using it, it takes off on its own.