Okay, I learned about "weak ties," the people who we know more functionally or formally than our friends and family, several years ago. Connecting with your weak ties is the best way to keep up with what's going on in the wider community, including business and job opportunities. Email, blogging and now social networking sites have made it much easier to stay connected with your weak ties, and marketers are dying to have product endorsements circulate through them.
Now marketers have to learn about "moral hazard." When your weak ties share information, they will be more or less free with the information depending on the risks of being wrong, of being "old news," or of increasing competition for a limited resource. (For example, I probably won't tell you about a job I'm trying to get for myself, especially if I think competition for the job will increase as a result.)
Search Insider: The Strength Of Weak Ties and Search, 2007-Aug-2, by Gord Hotchkiss
...a successful viral campaign is largely dependent on those weak ties being motivated to pass along the information. It needs to be remarkable in some compelling way (i.e. Godin's Purple Cow), it has to eliminate a scarcity mentality, it has to feel authentic and, to appeal to the mavens, it has to have the feel of news.