One part that particularly struck a chord was from a study that is being carried out by Cameron A. Marlow, a research scientist at Facebook, who has studied social media communications including wall posts, shared photos, pokes, and friend requests among 200 million people to determine how close we are to our friends online. According to the article "They looked at how often people clicked on their friends' news or photos, how often they communicated, and if the communications traveled in both directions. Studying this data, they determined that an average Facebook user with 500 friends actively follows the news on only 40 of them, communicates with 20, and keeps in close touch with about 10. Those with smaller networks follow even fewer"
The conclusion to this is obviosuly that in order to make sense of the opportunity to drive WOM then the focus should be on the closest friends ignoring the wider group - as they say "By focusing campaigns on people who interact with each other, they'll likely get better results".
I think the opportunity does differ by product as I know I have a small group of close friends with which I will discuss certain brands and products in detail I also have a slightly larger group of friends who are experts in certain areas - wines, holiday destinations, bikes, technology etc. BUT I also have a much wider group of acquaintances with whom I will discuss either very specific issues with eg a new laptop or music system (as they are perceived to be experts in these areas) and this is is where it becomes an awful lot more complicated.
I guess as a brand or company trying to tap into this the key is coming back to the traditional method of understanding your consumer and their decision making journey and where and what degree of influence the friendship group carries on this journey.