Most successful communities you see today began with a thin wedge.
The successful communities you see today began small. They focused on a single, tiny, element they could dominate. By focusing on a specific issue, or group of people, they could build up pressure.
Craigslist began as a mailing list. Craig would share notices of programming events in San Francisco. Soon all the programmers in the area had joined. It became a great place to find jobs or recruit job-hunters.
Facebook began with Eduardo’s e-mailing list from a single college.
When you launch a community (or almost anything), you need to know what your razor-thin wedge is. What is your single focus for now? Who, exactly, are you trying to reach? What, specifically, do you want them to do?
As you grow you can identify new opportunities and capitlize upon them. You can broaden your focus. But, for now, to break in to your audience, you need to find your thin wedge which you can dominate.