I think viral growth should be considered separate from word of mouth. And if you do make that distinction then there are very few products which became succes

I think viral growth should be considered separate from word of mouth. And if you do make that distinction then there are very few products which became successful through word of mouth. Original Google Search, Napster come to mind, and maybe Gmail (like the sibling suggests, though not clear if it's really a commercial success). Note that all of the above gave out huge value for free.