aristotle - 9:02 pm on Apr 21, 2011 (gmt 0)
I agree that Google is probably shifting to user metrics as a major indicator.
But I would say that Google's best source of data on user behavior is the Chrome browser. I think one of the main reasons they created it was to be able to collect better data. Tens of millions of people now use it as their main browser. That's enough to be able to collect statistically meaningful results. Some of the signals that chrome can provide included:
1. A user bookmarks a page as a favorite.
2. A user saves a copy of a pge on their hard drive.
3. A user returns to the same page later.
4. A user visits other pages on the same site.