Contact the opinion leaders in your area or startups with a strong brand who blog about the topics you want to focus upon. As previous poster pointed out, find communities with high-level of contributors that are actively engaged. Crosspolinate the best posts across other social media to present your community as a knowledge leader and leverage key words that resonate with your audience.

As per previous post, for active users there has to be something in it for them. Sometimes it is the quality of the discussion. Sometimes it is to build their existing brand as a knowledge leader. Perhaps you have perks through partnerships with other services. These are starting points. Good to constantly survey your existing base to understand what is working and what is not working with your existing service.