After rising steadily for many years spam levels have mysteriously dropped 30% in the first week of January. According to SoftScan the most plausible explanation is that a botnet has broken down and lost control of it's zombie computers.
Other possible explanations have been put forward including a large number of infected machines getting replaced by new computers received for Christmas or spammers being isolated by the Asian earthquake. Both explanations are considered unlikely.
Spam accounted for almost 9 of 10 emails in December.
source:
http://security.itworld.com/4774/070...de/page_1.html