When my site has substantial word-of-mouth momentum, then I'll try to accelerate it with other forms of marketing. But it might turn out that word of mouth is the only thing that works.
But how do you build that initial Word-Of-Mouth Momentum? Can't be easy to do.
Use it yourself. Get people you know to use it. Make it better until they start talking to their friends about it. (My project is not a social networking site.)