Botnets have rebounded for several reasons, most notably because they're profitable, said Stewart, who recently repeated his census of April to come up with a new ranking of botnets.

"Cutwail," the biggest beneficiary of the demise of Srizbi, took the top spot in Stewart's revised chart. It boasts an estimated 175,000 compromised PCs, up from 125,000 in April. "Cutwail's spam output actually increased shortly after [McColo], so it probably picked up some customers from other botnets," said Stewart.

As he did last year, Stewart estimated the botnet sizes by first "fingerprinting" each botnet with their implementations of SMTP (Simple Mail Transfer Protocol), then took a one-day spam traffic sample from each bot to extrapolate a total number of infected PCs in each botnet.

Although Rustock was also hit by the McColo shut-down, it was able to recover when Srizbi could not. Currently in the No. 2 spot, Rustock, controls approximately 130,000 computers, down from the 150,000 it "owned" last April.

Joining Cutwail and Rustock are a pair of new additions to Stewart's list. Until recently, both "Donbot" and "Xarvester" had been minor players in the spam-sending ecosystem. "We have noticed that some botnets picked up traffic significantly," said Stewart, who called out the two as botnets to monitor during 2009.