I'll provide one suggestion that should help. Make sure to acknowledge your users. This can be done many ways, but I'll give two suggestions.
If you spend a ton of time on this site in your first couple of weeks, or maybe it's a month, you'll get an email. The email will let you know that you have been one of the more active users over your first couple of weeks, and encourages you to continue. Acknowledging your top users and encouraging them to continue will help you build your base.
Another way would be to send an automatic email one month into some one using your system. Just celebrate the fact that they have been using your service for one month.
So I don't know exactly what you are capable of handling with your site, but this suggestion can be taken lots of ways. Just make sure you acknowledge your users, let them know how their content is helping to build a community in which they are in integral part.