The Stanford scientists pointed out that, because the study only looked at chain letters that reached a large number of people, it was impossible to see any letters that might have had a subcritical ratio of receivers to senders—these letters would have died out too quickly to achieve the wide-reaching spread that the earlier researchers wanted to study. It's entirely possible, the Stanford researchers noted, that the Internet may not be able to sustain a chain letter that follows the usual supercritical path where each person passes it on to more than one person.
via arstechnica.com