Making use of existing market data

Pew Research Center for the People and the Press -- Biennial Media Consumption 2004, 2006, 2008, 2010, 2012, 2014
http://people-press.org/dataarchive/

Newspaper Association of America -- Trends and Numbers: Newspaper Web sites (Reach by DMA)
(Click on menu items on the top and on the left for more stuff)
http://www.naa.org/Trends-and-Numbers.aspx

Newspaper/Magazine Print Circulation Data (click on eCirc)
http://www.accessabc.com/products/freereports.htm

ABC's Audience-FAX^* eTrends Tool:
The tool is designed to allow users to create trending reports by reporting period on newspaper's average circulation, average print and online readership, total combined audience, and total unique Web site users.
http://abcas3.accessabc.com/audience-fax/default.aspx

ABC's Audience-FAX online database (registration required, free):
Data on newspapers' average circulation, average print and online readership, total combined audience, and total unique Web site users as well as a variety of print demographic information for both national and local newspapers.
http://abcas3.accessabc.com/scarborough/login.aspx

Nielsen/Scarborough -- Free Reports
http://www.scarborough.com/freeStudies.php

Quantifying Twitter activity
http://tweetstats.com