I tried to build an online community of youths interested in politics and failed for lack of people. (We did have a very narrow target group: German youths who

I tried to build an online community of youths interested in politics and failed for lack of people. (We did have a very narrow target group: German youths who were both interested in social matters and enjoyed writing. That probably killed us, since we didn't (yet) manage to spread our advertising net wide enough to attract enough people for a snowball effect.) So I think you're first problem isn't going to be how to keep your community, but how to grow it. I'd like to hear what ideas others here have on that?