Every time a fan posts a song to her wall, all her friends see the song. They can listen to it, and from there they can check out your BandPage and give you a new like. Encouraging your fans to share your tracks is a rad way to get your music out to new ears.
So how do you get your fans to share your tracks? Just ask them! Make a call to action when sending out your songs asking your fans to share your song. Then watch your like count grow.