How do sites like this gain a "critical user mass"?

I know people who have started forums for things where no other forum exists to talk about those things yet they cannot get them off the ground. It seems there is a catch-22: No one wants to use a forum that no one else is using.

For highly technical subjects that a limited audience would be interested in, you can’t really get all your friends to support you because not all your friends are interested in it.

I’ve noticed that many successful fora provide a specific useful tool to the masses (e.g. Craigslist, Yelp, Fluther, Twitter, Facebook, Shelfari). But others for specific topics do exist and do have loyal followings. How does a forum become an internet staple and avoid getting swept under the cyber rug?