As I understand it -- somebody please correct me if I have this wrong -- the thing about Cloudbleed is that there isn't necessarily any relationship between the site whose page is cached and the site whose credentials appear in that cached page. So the only way to know that a particular site didn't have credentials leaked is to search all the caches of all the search engines on the Internet.

So, as perlgeek says, we'll probably never know specifically what the impact was.