To build this new version of Messenger, we needed to rebuild the architecture from the ground up and rewrite the entire codebase. We are excited to begin rolling out the new version of Messenger on iOS. To make the Messenger iOS app faster, smaller, and simpler, we rebuilt the architecture and rewrote the entire codebase, which is an incredibly rare undertaking and involved engineers from across the company.
Compared with the previous iOS version, this new Messenger is twice as fast to start and is one-fourth the size. We reduced core Messenger code by 84 percent, from more than 1.7M lines to 360,000. We accomplished this by using the native OS wherever possible, reusing the UI with dynamic templates powered by SQLite, using SQLite as a universal system, and building a server broker to operate as a universal gateway between Messenger and its server features.
Source: fb.com