Read the State Department's full announcement here.
"The U.S. Department of State will no longer add visa pages into U.S. passports beginning January 1, 2016. Previously, U.S. passport holders had the option to pay for the insertion of additional 24-page visa inserts when valid passports lacked adequate space for entry or exit visa stamps. The decision to discontinue this service was made to enhance the security of the passport and to abide by international passport standards.
To mitigate the impact on frequent travelers, the Department began issuing 52-page passports to all applicants outside the United States starting October 1, 2014 for no additional cost. Applicants within the United States may choose a 28-page or 52-page book.
Requests for additional 24-page visa inserts will only be accepted until December 31, 2015. Beginning January 1, 2016, applicants in need of additional pages in their valid passports must obtain a new passport."