At least as important as the online network, will be the offline network. I believe that the best kind of social capital is built from live (not virtual) interaction. So what I'm envisioning here is some sort of tool that enables you to get in contact with people who live within a certain distance of you. You could then contact them via DYC.org (that way people don't have to publish their personal information) and get linked in to what people are actually doing in your community.
My newest "aha" moment came today when I realized that the causes are going to be the link (insert "social capital" here) between organizations. People may be passionate about finding a cure for breast cancer, so they get involved in both the Susan J. Komen foundation and the American Cancer Society. They meet people in both groups, and rather than being simply the passive contributors we read about in Bowling Alone, we suddenly have active, collaborating groups, simply because they discovered a cause! This is awesome!
Access to Social Networks: What are the odds that we could partner with Facebook once we have this up and running? I'm imagining some sort of fb application that lets people show what causes they're linked to, or post information they believe in. Because people are already connected, it might be a way of taking fb and virtual networks and actually doing some good with it!