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So many sites were affected when mighty google did a major cleanup; they lost their pageranks and were penalized for advertising. Those who were on top of the list who were using SEO schemes to promote their websites were thrown to the bottom of the list. And sites that are selling links had to suffer as well. They too have been penalized because those links have been thought to be sold illegally. What a lot of people don't understand is that google is not against advertising. If you are a website owner and you offer to link up to other sites or products, you just need to disclose that you have been paid. Not disclosing is okay too, but you have to do nofollow links.