Social media is such a vast network that is intertwined from person to person. In your group, if you can reach just 10 people and they are interested in what you sell or write, they will all share it with their group of followers, who in turn will share it with their group of followers. Before you know it you reached around 550k people in record time. There is no advertising or other types of network that can reach that many people in such a small amount of time. The social network is a grapevine to reaching people that you could never have reached on your own.
If you have a Twitter account you can start off with small tweets that will get passed around from one network following to another. You can build up your product and get people excited about the release date. This is better than spending millions of dollars on advertising and not reaching half as many people.
Social is free and people love to share. If you own a blog or a store it is your best chance to spread the word and get people interested in what you have to offer. Furthermore, it is cheap and won't cost you anything. Just a little bit of your time.