On a proposal page over at Area 51, it says,
Eventually, 90% of a site's traffic should come from search engines.
I propose it would be useful to add a referrer row to the metrics table. I don't suggest anything too low level, more along the lines of:
20% Search Engines (Bing, Google etc) 15% Social Networks (Twitter, Facebook etc) 45% StackExchange Network 20% Other
Far form just stats porn, it would provide insight into the quality of content and dynamic of the site. Low search engine referrals would indicate poor quality questions and answers or little wider appeal. High social network referrals suggest active and engaged community.