20.6 million people watched President Barack Obama’s second inauguration on television, down 46 percent from the 37.8 million who watched in 2009, according to new ratings from Nielsen.

http://www.politico.com/blogs/media/2013/01/inauguration-viewership-down-from-155026.html

http://www.latimes.com/entertainment/tv/showtracker/la-et-st-obama-inauguration-ratings-tumble-from-first-ceremony-in-2009-20130123,0,4452171.story