The new word of mouth.
Back in the day, many businesses relied on word of mouth for advertising. There’s nothing more effective than someone you know giving a personal recommendation for a product. It has always worked fairly well. Someone really loves your product, so they tell their friends and those friends tell other friends, and it continues on. (Like the old Faberge Shampoo ad. “They’ll tell two friends, and they’ll tell two friends…”).
The process is still the same, but the communication methods have changed. Word of mouth is still there, it just comes from a keyboard now. Instead of telling two friends in person, they’ll hop on Facebook and tell 135 friends. (And they’ll tell 135 friends…) There is the potential for you to be reaching a massive audience with Facebook without doing all that much work to get it.
Even if you don’t have an official Facebook presence, you should at least think about monitoring the mentions. People want to share their great new discoveries with people and social media makes it much easier for them to do so. You might not be ready to (or even need to) jump right into the world of social media, but you should at least start listening.