This is a short, but good one. Social Patterns blog has a post that appears to logically detail how Google determines whether or not to give a site extra "snippet" links in the SERPs.
Summary:
Google snippet links are most likely determined by traffic patterns. Since Google does not allow access to toolbar traffic data, there is no way to know for sure. Many site’s snippet links closely resemble Alexa traffic stats.
Worth a read - great detective work, Michael!
Does anyone have any new information regarding the algorithm for the choice of snippet sub-links? It looks like the last anyone weighed in on this topic was 2 years ago.
"My feeling is that Google is more interested in tracking surfer patterns in order to assess how popular sites become popular."
I agree. I just posted Big Daddy - Seeing Anything Different" and am wondering if some of these metrics are creeping my SERPs upwards. I have one page that has slowly floated from #10 up to #1, passing some good .gov and .edu pages - but I am betting that mine has the most engaging content. What makes me curious is... This page has zero external links and only one internal link.
There are probably multiple factors that are combined to determine which links to show. The pages that appear in the links that show for my site are....
* persistent navigation for the entire site * heavy traffic * gateways to rich content (either onsite or link-out) * hold #1 or #2 for their main KW
Exactly correct Michael - guess I didn't make it clear enough about the distinction.
My article discusses specifically the snippet link (sub-links as you call it), not how Google returns the UI snippets. Google determines which sites to return the snippet links algorithmically.
From the snippets I see for my biggest site its definately not linkage data. It matches the traffic data almost perfectly though.