I only know of four viable methods:
- A simple core built-in feature, where by using the product, users invite / share it with others. This used to be mostly about email address import, although these days Digging, Tweeting, and of course, spreading on Facebook are very popular
- Users embed the product onto their own blogs / pages / sites
- Users create original content that gets picked up by GoogleBot, gets SEO’ed, and then brings in other users via Google search.
- Users actually pay for your content, whether through subscriptions, or much more likely, virtual goods. This, in turn, allows you to buy traffic
If you don’t have one of these four methods in operation, and you’re building a web-based company, you’re going to find yourself begging for traffic a lot.