According to an article in The Guardian last month, the answer is developing your marketing strategy, and in particular, your social media strategy.
Why is this? Well, in the current age of needing to get information out to consumers fast, you need to give thought to who you want to sell your products or services to and how you are going to get the information to them. With so many people accessing a social network on a daily basis, using Facebook, Twitter, Pinterest, etc, is an effective way of giving your target audience the messages you want them to receive.
By having a strategy in place from Day 1, you can control these messages and, according to the article, see greater growth than you might expect without the use of marketing and social media.