Google's study shows that it tracked 760,935 hijacking incidents during July, 2014 - June, 2015 via its Safe Browsing and Search Quality programs.
This is a huge number of incidents!
One of the issues the study goes into is how well the incidents were managed, and 80% of cases were cleaned up after the first alert from Google. Surprisingly, it indicates that although the site was cleaned up, the root cause is not addresses by as many as 12% of the sites getting hit again within 30-days of the initial incident.
Here's the full study, in the form a PDF.