One of my sites is in top 5 for certain keywords. When I was using AdWords for exactly these keywords I was getting certain amount of impressions. Now I don't use AdWords for that site and I'm getting less than 10% of impressions from search engines compared to what I had from AdWords for each and every of keywords. I use Google webmaster tool to see impressions statistics. This is very disappointing, considering the efforts made to push the site to the top 5 of Google. No change was made to a website. Can someone help me to understand why is this happening?