SE Roundtable published results from a spam research. They report 77% of all Google BlogSpot blogs are spam. Other blog services don't score much better, 74% of all Netscape blogs is spam and on AOL's blog network it jumps from more than 80% to more than 90% on some domains.
...14 of the top-15 doorway domains have a spam percentage higher than 74%; that is, 3 out of 4 unique URLs on these domains (that appeared in our search results) were detected as spam. To demonstrate the need for scrutinizing these sites, we scanned the top-1000 results from two queries – “site:blogspot.com phentermine” and “site:hometown.aol.com ringtone” – and identified more than half of the URLs as spam easily.
Another interesting snippet is the analysis of which top-level domains have most spam: .info seems to be the worst with 68%, followed by .biz with 53.
The more popular .net, .org and .com TLDs score a bit better with a spam level of 12%, 11% and 4.1%, respectively.