Snowshoe spam took #1 in Canada again, through AS 32613 IWEB-AS, on the May 2012 SpamRankings.net. That was the first week of a spam plateau per ASN. The next week saw a platau for AS 33139 CANACA-210. And the next week it was AS 6407 PRIMUS. Canada, land of spam plateaus! Does this mean spammers are shifting from ASN to ASN for successive weeks of spam campaigns?
The old-time winners, AS 6327 SHAW and AS 577 BACOM, kept spamming away, and came in #2 and #6 again. That's in the rankings from CBL data. In rankings from PSBL data, IWEB, SHAW, and BACOM were #1, #2, and #3.
The rest of the top six were upstarts, not much seen until recently. Iweb did make a bid for the top back in September 2011, but its recent predominance dates only from February of this year.