Alexa provides a new and potentially interesting visibility metric

Alexa (an Amazon company) used to return the number of entries found in the top n sites. I routinely queried it to get the number of mentions for specific concepts appearing in the top 100 sites. 

Since April 1st they now return the number of sites "about" a concept, within a user defined bracket running from the top 1 to 10M.

Their FAQ indicates that their crawler is designed to find sites rather than documents. There is no doubt that Alexa was doing a rather poor job at indexing documents. On March the 31st they returned 337K documents pertaining to "Barack Obama" vs 4,140K pertaining to "John McCain", a rather surprising ratio of 12-to-1 in favor of McCain. (Google returned 172M mentioning Barack Obama vs 34.5M mentioning John McCain)

Alexa's new site search returns a plausible 3,000 sites worldwide "about" Barack Obama vs 176 "about" John McCain. 

A very preliminary investigation raises questions about the value of such information (Alexa reports close to 52,000 sites about the iPhone…) but opens tantalizing opportunities such as finding if sites about a given concept are more of less prevalent as we move up in the rankings.