I have noticed the big difference in stats also. Mine started on January 17th, I lost 1/4 of my hits. Then on February 8th, I lost another 1/4 of hits. So, I lost 1/2 of my hits. Although, this did worry me some. My stats now are more inline with what Google Anaylitics reports for my traffic. Before, I was starting to believe, I was just getting this magical 14,000 visitors a day that Google didn't detect.
My free sign ups and revenue has went down some, but nothing that couldn't be just normal activity.