Eric Ries defined three different types of growth engines in his book The Lean Startup.
Marketing gurus like Seth Godin will tell you that the Viral Engine is the most critical one, that is, people spreading the word about you. The more you know your audience, the better you can tailor your content to be shared.
In theory, that will lead to organic growth: people will google about you, retweet your tweets, share your facebook posts, and many other untraceable actions. Unless you are a solopreneur, looking to make a living with a handful of clients, if you seriously want to scale your business, you need a paid engine of growth.
It means paying Google or Facebook to place ads in front of potential customers. You are paying for attention. In the viral engine, you don’t have much control about what content is being shared, and to whom. In the paid engine, you know exactly to whom you are promoting your product. It is more expensive, but it is faster and more accurate.