Before You read my answer - consider asking some who has never looked at your site to walk through the user engagement process. Is it warm, simple and inviting? What is the web traffic through the signup process - do people get there? Where do they fall off?
Next - once you are sure that the process is clean, simple and inviting:
Find a seed audience. Convince that audience that you can help them achieve their goals.
In your case - the first place I would go after are community activists. The world is full of "communities of common interest" that are looking to spread their message.
Find a small group of them, and turn them into your evangelists with their own ideas and markets.
I could give you a list to get you started if you would like.
I also have some rough notes that I could go over with you on how these things tend to grow in emerging areas of interest. I would also consider how you can engage Geographic interests into the equation.