Facebook removed the mobile format of Instant Articles from Messenger. A Facebook representative said that the company will develop the service only in the main application.
Instant Articles were launched in July 2016 and firstly they were only designed to be available in News Feed. Later the platform was expanded to Messenger in order to provide more features to Messenger users.
In May 2017 Facebook introduced a new feature for developers. It allows creating and publishing content in 3 formats simultaneously: Instant Articles, Google AMP and Apple News.