The big benefit of word-of-mouth is that your marketing budget goes toward zero, as your users become your marketers. If theyre so passionate about your design theyll tell their friends about your service, and you wont have to. And, most likely, what they say is more influential than what you can say anyway. Focusing on this value, and designing to enable it, is a big part of social design.
Word-of-mouth is complicated from a design standpoint because its not a monolithic activity. Its several smaller steps that happen in order. On one hand this makes it harder to design for because there are many little problems to solve. On the other hand, it gives designers a clearer picture of what to focus and spend time on.