From Ars technica:
Security firm 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.

