The Department of State announced that, as of January 1, 2016, it will no longer be possible to add visa pages to U.S. passports.  Instead, visa applicants requiring additional pages in their U.S. passports must obtain a new passport by mail.
The new policy replaces this previous ability of passport holders to purchase a 24-page insert for their passport in order to accommodate additional entry or exit visa stamps.  The change is to improve the security and integrity of the passport document.