Here's a good article by Howard Kingston, outlining tangible ways to grow a product to millions of users, including viral marketing, facebook marketing, localisation, open graph, cross-promotion, and discovery. A good overview of modern mass marketing techniques, basically.
The techniques are declared to apply to both games and other products, but caveat emptor: many of those won't work unless you are selling something to a mass market - in other words, anyone could be your client. Niche products beware.
