The Four Horsemen of Usability
http://webword.com/moving/fourhorsemen.html

Rhodes, John S.
WebWord
2001

Abstract:

As of June 2001, four web properties control more than 50% of all the time spent online by U.S. surfers. This means that you can throw away your usability guidelines and follow these companies. They spend millions on usability testing and they are driving standards by sheer market force. You have no choice but to follow their lead.