From a post on the Facebook Accessibility team's Facebook page:
Supporting Dynamic Type on iOS has been in development since the very beginning of the year and turned out to be a challenging technical implementation. Since text enlargement affects every part of the app, we needed to make sure that many key features supported the change and that layouts weren't broken (resizing text pushes things around on the screen). In order to accomplish this, we built a general framework for all product teams to use. We then worked closely with a few product teams to adopt the system and refine it. After getting Dynamic Type into many key products, we also ran many rounds of testing to make sure things displayed properly on all possible settings and surfaces. The final step involved a small test release to the public and a bunch of fine-tuning to ensure the best possible experience. We hope you enjoy the new feature and welcome your feedback as always!
This is great news. I've been waiting for Facebook to do this for a long time.