It's a self-reinforcing thing. The better the traffic, the more motivated the developers and the users. And the more motivated the people are, the better the site gets. And the better the site, the more traffic.

Community sites take time to build. They need to be carefully nurtured. But once they get going they feed on themselves and then become unstoppable. Look at Facebook as the classic example. Google Plus is the superior product on a technical level, but it came too late and Facebook already had an unbeatable momentum.