Forums, and hosting forums within my own for official band sites, which helped bring in about 80% of my user base. And they, in turn, bring in other fans, suggest other bandsites to offer forums to, etc etc.
I started my community one year ago - now its the largest mountainbike-related forum in Germany
How: I e-mailed many other webmasters having mtb-sites about running a forum together, because everyone had their own empty forum ;-) If they joined my forum they would get special rights etc...
I dont have an own mtb-homepage anymore only forums!