Marketing is where the real business is - these days anyone (thanks to boonex and others) can have a social community site, but it won't be a success or have any members without marketing.
Advertising is the quickest way to bring members, and it's relatively inexpensive these days. Choose Google ad words or rich media solutions from places like doubleclick, rightmedia etc to target both the kinds of users you are looking for and the sites they frequent.
PR is cheaper, of course, but harder to do. For PR you need to announce who you are and what you do to an audience that cares. That last bit is the key - you can shout from the highest tower that you have a new site, but if you're not talking to people that care it doesn't matter. Find some blogs, for instance, that serve the community you want to attract and try getting some PR there.
Promotions are a sort of hybrid of the two above strategies. Typically a promotion involves giving something away for free (ipod, car, lip balm, whatever) in exchange for signing up. While many of the people you attract might not really be interested in the site, at least it brings in people. If you hold a promotion you should announce it to the people who care the most - either those who care about the prize (blogs about lip balm, for instance) or to the sites that engage the community you want to attract.
Other simple ideas are link sharing, referral programs (where you pay people who give you business) and also in-person marketing (actually visiting people or companies that are in the community you want to target).
It's not easy, but it's the key to success. If you want to get a real sense of how it works you should get a few books from Amazon or the local library;
Gorilla Marketing
How to Use the Internet to Advertise
New Rules Marketing
eBoot Camp Marketing