-Do you have a blog? Starting writing. Share the contents via RSS, Twitter, FB etc.

-Do you have a twitter account? Start following relevant people, get involved in discussions, share your link (don't spam).

-Write to bloggers/journalists who might be interested in covering social networking sites (mashable?)

-Since it's niche, promote your site on every blog, forum, site related to the niche. Again, don't spam but inform others why its to their benefit to join your network.

-Adwords?

EDIT: The site wasnt loading when I tried, so I wrote all of that without visiting the site. But now that I've seen it, I'm not really sure how this is a "niche" social network at all. Confused.

-Some sort of content where existing members can win prizes for referring others?