This is a pretty good post by Kevin Rose on how to take your website to a million users.
http://highscalability.com/blog/2009/10/6/10-ways-to-take-your-site-from-one-to-one-million-users-by-k.html
To sum it up:
- It should feed the user’s ego. Top scores, Twitter Follows (how many followers do you have?)
- Focus on 2-3 items that your site should do well
- Release often. This means that your operations/process has to support immediate feedback and release
- Leverage on your user base
- Provide value to 3rd party sites and leverage on their traffic
- Market through unique techniques like Invite Only signups (eg: Gmail in the early days)