In the comments of Scoble's post, someone asked “how does word of mouth start?” If you're interested in the answer, read The Tipping Point by Malcolm Gladwell.
I've been meaning to pick this book up for awhile, and finally did so on Thursday. Unfortunately I'm a bit bogged down with work right now and haven't had a chance to finish it, but it's been an excellent read thus far. 'The Law of the Few' is especially relevant in the word-of-mouth discussion.
The basic premise is that there are three classes of people who make up a very small percentage of the population but are largely responsible for the spread: Mavens, Connectors, and Salesmen.
In a social epidemic, Mavens are data banks. They provide the message. Connectors are social glue: they spread it. But there is also a select group of people - Salesmen - with the skills to persuade us when we are unconvinced of what we are hearing, and they are as critical to the tipping of word-of-mouth epidemics as the other two groups.
One of the most interesting groups to me (and in my mind, perhaps the most important) are the Connectors. With many 'weak ties', they literally connect disparate social worlds.
The premise here is that the majority of us are mainly concerned with our 'strong ties' - our close group of friends which is typically homogenous in some sense: location, activities, and so on. Telling all your close friends doesn't create an epidemic, because typically that is a closed group. Connectors are powerful in that their "weak ties" make them a gatekeeper of sorts between the different worlds. It is the Connectors that facilitate the flow of information between the groups.
The author mentions a study by a psychologist named Millgram in the late 1960s in which he mailed a packet to 160 people in Omaha, Nebraska. The packet contained a chain letter and the name and address of a stockbroker who worked in Boston and lived in Sharon, Mass. The people were instructed to send it to a friend or acquaintance who they thought would help get it to this stockbroker. Millgram found that most of the letters reached the stockbroker in five or six steps (giving rise to the concept of six degrees of separation), but also that half of the responses reached the stockbroker through three people.
The important point here, of course, is that not all degrees are equal.
You can pretty easily verify this if you belong to any of the social software communities. As I look at my network of friends, for example, I've noticed that many of my connections involve Robert somewhere in the chain. Dave Winer is another good example - he bridges several distinct groups (legal and technology being the most obvious example).
Anyways - the point is, this book does a pretty good job at explaining how epidemics (social or of disease) happen, so if you're interested in the answer it's definitely worth a read.