Let's face facts: delivering content to users is the primary goal of the web. If an application is primarily based on a social function, the content has to be provided by users - a commodity most new applications don't have. Here's a short guide I've written based on my observations and experiences on kickstarting a new web app.
Even if you haven't launched a product, prep your potential user base with a few simple e-mails and a promotional site. You've seen these sites before - a simple description of the product and an e-mail submission form will do. If you can afford it, have a professional designer create the branding for your app. It's much easier to get someone's attention when something looks good.
Of course, without traffic any promotional site is useless. There are a few simple ways to generate traffic (and buzz) for your new site. Start with something simple - e-mail bloggers, writers and even reporters describing what your product does and how to get on it. Ask them politely if they would like to try it or even write about it.
You can post your promotional site to del.icio.us, Digg and other social bookmarking services, too. They can act as good indication of what the general feeling is regarding the app (check the comments section). If a particular discussion forum could be interested in your product, find out if it's okay to post about it and follow up - use discretion when doing this though, it could always backfire if you're too bold.
Depending on the product, it may take a while for the traffic to start flowing in. Keep an eye on the server statistics and check referrer URLs - perhaps even e-mail some prospective users or reply to a comment posted on a blog.
Once there is a significant traffic boost to the promotional site, you're set. The key now is to retain users, primarily by providing new content of your own creation. I'll discuss this in a little more depth in another article - the general idea is that when you actively give back and participate in the community, they will become more loyal. This is important for the viability of any social app.
So don't stop promoting your product after users begin to sign up. If you're actually making money from your app, look into ways into expanding the user base even more. Maybe you could post them here.