Users bring you traffic
Duh.
But how?
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. More