As many have pointed out, Google has used clickstream data from their toolbar for a number of years. Also, Google has used the links provided by hub and search

As many have pointed out, Google has used clickstream data from their toolbar for a number of years. Also, Google has used the links provided by hub and search pages to find relevant sites within a niche. They have happily indexed links they discovered on those pages, and then removed or penalized the pages that pointed them to it. It's OK, of course, because any SERP not provided by Yahoo Google or Microsoft is termed "spam"