According to security firm Marshal just six botnets are responsible for 85 percent of all spam:
Srizbi is not alone in the post-Storm botnet world. Marshal estimates that six botnets account for 85 percent of the total spam sent world-wide. Srizbi is currently in the lead, with 39 percent of the "market," followed by Rustock at 20 percent, Mega-D at 11 percent, Hacktool.Spammer at seven percent, Pushdo (6 percent), and Storm (two percent).
These numbers track the amount of spam each botnet is producing rather than the total number of systems infected by each botnet. The two numbers can vary widely; At its peak, the Storm network accounted for 21 percent of all spam and contained an estimated 85,000 bots. Mega-D, on the other hand, grew to encompass 32 percent of the spam network in early February, but contained only an estimated 35,000 bots.
One interesting development that may also provide further proof that the botnet industry is commercializing is the mounting body of evidence that suggests multiple botnets are being used to advertise a single product or group of products. Over the past few weeks, Marshal picked up advertisements for the "Express Herbals" website from sources infected by Srizbi, Rustock, Hacktool.Spammer, and Pushdo.