Time was, ranking was all about the links. PageRank, link popularity, reciprocal linking, triangular linking, mini-nets and link farms. These catchphrases dominated every SEO's conversation. Google stored all their pages in one big index, and we all waited around for the monthly Google "dance" when Google would update that index and we'd get to see the latest results of our optimization efforts.

But times have changed. The web has gotten so big that Google's gone and split their index in two: One index for the pages that get ranked, a second supplemental index where they relegate all the second-rate junk they don't care about. These supplemental pages are technically included in Google's index but have no chance whatsoever of ranking well for any meaningful search.

While it's still true that getting ranked continues to be mostly about the links, these days the advantage to getting high-PageRank links is less about the rankings and more about staying in Google's good index—and out of the supplemental dregs.

Today's poorly-linked sites are getting crawled shallower and shallower, with Googlebot hardly ever visiting many of the deeper pages within the site's subdirectory tree. And, as more time passes without Google indexing those pages, they get closer and closer to falling out of Google's main index and into their supplemental index.

Google Cache Date has Become a Major Indicator of Site Health