According to the study (PDF link):
...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.
The reason for this is the suspicion that the popular blogging service is free. One WebmasterWorld member states:
The trouble is, there's no algorithm that can automatically factor in the price of a service. It's free to set up a blog on Blogger, so it can be abused more easily. If these spammers actually had to pay for a new domain name every time they set up a splog, they wouldn't bother.
Other findings of this research showed the spam percentages for Top-Level Domains (TLDs):
- 68% of .info TLDs are spam
- 53% of .biz TLDs are spam
- 12% of .net TLDs are spam
- 11% of .org TLDs are spam
- 4.1% of .com TLDs are spam
Forum discussion continues at WebmasterWorld.