‘What’s being called the Unnatural Links update is rocking the SEO world. In March 2012 people started receiving warnings from Google about “unnatural links” and then many of those sites’ rankings in search results have taken a nosedive. Some sites have been de-indexed, which means their site’s pages no longer show up in Google at all.
‘The scary part? Unlike other Google algorithm changes, you can’t respond to this one by changing things on your site. This time, it’s about links to your site from other websites (commonly called backlinks) that you most likely don’t have control over – including sites you may never have asked to link to you…’
Make sure you don’t have links from blog networks