Google is punishing websites with Artificial links or paid links. Many websites with high PR has seen drop in their PR.

I found this article very interesting and informative

Google Purges The Payola - Forbes.com
Googlers like to think of their company as the Web’s library. But as Web sites come up with more ways to wring profit from online advertising and sales–often using methods that Google opposes–a better analogy might be the Web’s watchdog. In recent months, that watchdog has barked at sites it accuses of shady practices–and it may be start

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