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"We're also looking at some ways to go upstream to deny the value to link spammers, some people who spam links in various ways...
How would they know?
All linking pages are subject to changing conditions upstream of them... anything from code changes to business changes to algorithmic changes, all of which can positively or negatively affect inbound link juice. Some pages have good enough content to attract inbounds once they become sufficiently visible, which can mask other upstream changes. In some market areas, links will be affected by upstream link buying and selling as well....
I'd like to see a much more granular exploration of those who were affected, as I'm thinking that the poll may also be skewed by types of sites and types of SEOs.
I'm suspecting in many cases that upstream link sources have gotten hit... and I'm guessing that the further away from the SEO industry the link sources were, the less likely it is that they were "tainted". This is completely gut level... I haven't done the kind of study necessary to be more sure of the causes than this educated guess... but from what I've been seeing, there haven't been many surprises.
Matt's comments are not about your browsing history. This is about following risky advice. If you trade black hat tips you are very likely to be utilizing some risky black hat technique on your sites. It is not surprising that Google penalties will eventually catch up with your risk taking.
Actually I think they get that first comment on your site pointing to another site and that site points to their main site. This way, if your page gets penalized, they break the chain and protect their main site. You get all of the ranking problems and the chain no longer reaches their main site so theoretically they don't lose rank.
It takes a lot of energy to be lazy! Thankfully Matt Cutts suggested that these 'tiered' linking methods will meet their demise later this summer in an update he expects all SEO's to talk about.