Alright first of all, the idea is great. If you read Dharmesh's blog, you'll see how much value a(an) UX/UI/Designer has. You won't need a perfect marketing plan if you have a perfect design, and user interface. One that promotes accessibility. I voted your question up because its a legitimate question and you have a great idea.
Also, I see that you design is really really, simple. Its a good thing. I like how minimalistic and to the point this thing is. I really do. I looked at your sourcecode and I see tonnes of external style sheets linking to one page. I recommend you copy and paste all the code into one main css. This will ensure you don't cost yourself a lot of bandwidth.
Anyways, lets get to the point. Anything I tell you is just my opinion, but I can say it'll work for you. First of all, you need to work on social media marketing. We're in web 2.0 age..meaning cool looking pages, css gradients, html5, css3 and so forth. Think of Social Media marketing as the new way web 2.0 version of marketing. You probably won't need to do SEO if you do proper Social media marketing. I suggest you read Inbound Marketing by Dharmesh Shah. It'll get you up to date with SMM.
If you can't buy the book, to summarize (this isn't simple to do), you'll want to basically go where your potential customers/users are, since this is like a get together thing, you can just visit Social websites like Myspace, Facebook.. Go there and start making noise. Think of your self as a monkey owner who makes his monkey do tricks, instead of doing this in person.. you are doing this on the internet. Attract potential customers on those communities and revert them to your website. It is important you be as transparent as you can while you do this.
Anyways, I can't cover everything here. Words can't describe this. You'll want to research Social Media Marketing.
Also, if you need help with your website (Which, you do need - sorry if I am rude) let me know. I can help.
Resources if you want to improve your website
First of all, the market you're entering is by itself, troublesome - you're entering the misty and mystical land of Metcalfe's law, the chicken and the egg rule apply here; You can't make a service which is users' dependent work without users, and your service is less valuable if you have less users.
And thus, you have to understand that it's going to take work in order to penetrate that barrier, even a quick look at twitter's history when it was launched in 2006, reveals that it took them a long while to get enough user base and virility to capitalize on it, If you already had a working service which generates traction, you could launch a service such as this one more easily, but don't let this discourage you, it's just a hurdle which is to be expected.
Although my experience as far as social media is limited, I saw two things in your site which would discourage people from registering to it: