Spam data cited here is drawn from the network of Google email security and archiving services, powered by Postini, which processes more than 3 billion email connections per day in the course of providing email security to more than 50,000 businesses and 18 million business users. “In 2009, security community started seeing diminishing returns from the takedown of malicious ISPs. After ISP 3FN was taken down, spam levels rebounded in less than a month, and after Real Host went down, spam volumes recovered after only two days. In response, anti-spam community turned its attention toward taking botnets offline instead. Toward the end of 2009, Mega-D, a top-10 botnet – responsible for infecting more than 250,000 computers worldwide – was severely crippled through a carefully orchestrated campaign designed to isolate command-and-control servers spammers were using to support botnet. In early 2010, security professionals, along with government agencies, successfully mounted a campaign against several more targets: major botnets such as Waledac, Mariposa, and Zeus were either shut down or had their operations significantly curtailed,” notes Google.
More info: Q1'10 spam & virus trends from Postini