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What happens when you grow so fast that you start to saturate the population. This has happened to several Facebook app developers. They experience very rapid growth, and then suddenly the growth dies. Andrew Chen has written a great blog post about this:  Facebook viral marketing: When and why do apps “jump the shark?”. (Side note: I don’t believe that the equation that Andrew puts forward for simple viral growth is correct, as it assumes that the entire population will continue sending out invitations at each viral cycle. However his work on saturation of the population is very relevant for highly successful viral apps.) In case you are interested in where the term “jump the shark” came from check this out: Wikipedia: Jumping the shark.
Word-of-mouth marketing is a key component to the growth of a small business. It's often word-of-mouth marketing that keeps small businesses running in the early days of operation when there is little to no marketing budget. The consumer shares their experience with your products or services, and they share it with their family and friends. It increases your consumer base and increases your sales.

The reason to make use or virality, the ease in spreading and sharing, is however a double-edged sword. We cannot forget that in this type of campaign, a large part of the control falls into the hands of the users, and we risk the message being misinterpreted or parodied. On the other hand, a successful viral campaign can work miracles for your brand’s results.