freejung - 9:03 pm on Jul 28, 2010 (gmt 0)
I suspect links are weighted by actual traffic they deliver, measured via the toolbar and analytics.
They might be using clickthrough in the SERPS. Many have suspected that, though it has not been demonstrated to my knowledge.
It is unlikely they are using toolbar and analytics data as anything more than a cross-check, the data would be too limited for general ranking purposes.