Every entity represented in Twitter’s API has an ID associated with it. As we allocate larger and larger numbers for these IDs, the space required to store them must grow in turn. For example, we migrated Tweet IDs from 32-bit to 64-bit integers back in 2009, then did the same for direct message IDs in 2011. We expect to start allocating user IDs in a 64-bit integer space sometime this year. Update: This change is happening on October 21, 2013.
This means that if you currently use signed 32-bit integers to track user IDs in your system, you should update your code to use 64-bit integers as soon as you can. To be safe, and to provide a date to aim for, we recommend making this change by July 1, 2013. This includes statically typed variables in your application code and column types in your database code.