I check
www.KillerStartups.com every day. They are an excellent source of new and exciting startups. One thing you can do to market your startup is to do a Google Blog Search with various keywords like "Startup" or "Best Startups" and find all of the blogs that continually post about new startups. Pay for a professional press release, join a bunch of forums and change your signature, and pay for advertising on applicable forums. If your social networking platform has any mobile aspects to it, Google is beta testing mobile adsense right now and it could be mainstream by the time you go public. The competition might not be as heavy there. A forum member, Michael, has a mobile advertising platform as well, search around the .mobi forum for that.
http://startup.wsj.com/howto/marketingsales/20060703-needleman.html
That is a story about a startup that used MySpace spam to get some visitors. There are more graceful ways of going about it, but if you think about it, what better place to advertise a new social networkings site than existing social networks?
If you have a high budget, try thinking outside the box and do something that is completely weird and therefore newsworthy (like paying a dog kennel to shave your site's address on the side of all of their dogs.)
Don't just stick to web advertising if you have the budget. People still look at billboard signs. If your social network has provisions to make the site safe for kids, advertise at places where mom's frequent. You can also advertise on sites like MapSexOffenders.com or FamilyWatchDog.us for this purpose. Offer free tshirts to college students and advertise in college newspapers.
There are a ton of articles on Startup marketing too, and I'm assuming you've done the basic google search since your startup is "decent sized" and almost near completion.
Good luck! I hope at least one of my ramblings help spark an idea.
