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hmm, i'm not sure what you mean by google impression reports... from Analytics? or Webmaster tools? Either way, your backlinks still seem to be present. If you had been penalized you would usually see a drop in traffic for specific keywords.

You need to drill in to your data and see where the drop is coming from: Is it just search engine traffic or other sources (other sources as well, suggests either a market wide trend or on-site problem) If search, is it across terms or just a few (just a few suggests ranking drops)