Marketing a social networking website?

I've been struggling with getting a word out about our new social media website.
Functionality wise it's similar to the popular social networking sites but targeted to a specific niche.

All my experience has been building backlinks to blogs, service or review websites.
Marketing a social media website is very much different from that.
I can't focus on building backlinks for specific articles and optimize those to have decent search volume.

What we have been trying is getting the word around using reddit, twitter and facebook, submitting press releases and posting to various forums/blogs
but we are only getting about 20 uniques per day.

Currently we are doing some social bookmarking and it's not getting us anywhere.

Would really love to get it to sites like mashable or huffington.

How would you go about promoting such a website? Would love to hear your thoughts.
Thanks

dude
dont waste your time - unless you have $XX,XXXX

sorry to say but world is not ready for another twitter/facebook etc etc, Focus on your expertise - invest in a niche build links , rank and bank

We already spent around 100k creating the company, hosting the site, paying the developers, legal department, designers and so on.
It's not based on any shitty php systems.

wow - man - I have lost of words now

You should have a few $$ left for marketing. Get into some paid marketing like media buys or PPC.. the website should reach that viral point where it gets users daily from people already members.. but until you reach that point, you should at least try to get some paid traffic