article marketing could help. write some article in the same niche. then make list of related social sites and submit on them. there are other ways of course.
Aside from using other social networking sites, why not bookmark your social network, I'm pretty sure it will attract potential users.
Article marketing seems like a good idea to not only get some backlinks but also to get some decent traffic..
I'll just be realistic, you have to market it to a specific audience. If you are going to go out there and try and compete with Facebook and MySpace you might as well forget it. Their budgets and the time they've been established makes it too hard to compete in that arena. The only social networks I've seen do well are those tailored to a specific group. For example, on the television a few months ago I saw someone who created a network tailored toward those who are physically handicapped. His site was doing very well.
I agree with PHPGator. Once you've picked your niche (if you don't have one already), you can do some good old SEO for keywords related to this niche. For example, I know of a social networking site for emos that does very well because it ranks high in the search engines for various keywords and phrases related to emo. The reality is that very, very few people are searching for "social networking". They are searching for specific niches and may then find your site. I notice some people here are really big into writing articles. This is all good, but don't expect to get a lot of traffic from these articles unless they are so good that they are published on many high traffic sites.