Make your product stand out so that it spreads via word-of-mouth. I know it's an overused catch phrase, but if you're good enough that people want to tell others about what you've done unbidden, then you've got it made.

Just keep focusing your efforts on your product. Make it shine. Make every single part of it wonderful. People will talk about it. That's what happened with Facebook: I remember getting an account when high schoolers were first allowed, and feeling the need to tell all my friends. It had such a wonderful interface. Most of my friends did the same.