Word of Mouth at its heart was literally one person passing on comments or recommendations to another. (For great read on WOM fundamentals, check out this book by Andy Sernovitz). This required the physical sharing of a common locality to experience this Word of Mouth phenomenon.
As communications technology advanced, this spatial requirement for Word of Mouth diminished. Enter the Internet age – where we have people from one part of the world influencing others on the other side of the globe.