A sustained, non-booming, viral growth is a great way to reduce marketing costs by letting the customers do the marketing for you!
Reference
Ries, Eric. The Lean Startup: How Today’s Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses. 1st ed, Crown Business, 2011.
Highlights or timestamps
The viral engine works by getting existing customers to take care of the company’s marketing. Awareness of the product spreads among your target customers by word-of-mouth. This can save you a lot of marketing expenditure, so you should make it as easy as possible for customers to engage in this kind of viral marketing.
— ^e5bb7b from The Lean Startup