In brief: Remember when you could chat to friends in the main Facebook app without having to load Messenger? Five years after it disappeared, it looks like that option might be returning.
App researcher Jane Manchun Wong, who has spotted numerous features being tested on various social media applications in the past, discovered the change.
Messenger arrived as a standalone app in 2011 and was removed completely from the core Facebook app three years later.
Reverting back to the old format would see the Messenger icon remain in the Facebook app, but instead of opening a separate app, clicking on it takes users to a 'Chat' section.
Right now, Chat only supports basic text and lacks many of Messenger's features, including making calls, photo sharing and reactions, though this could be because it's still in a testing phase.
Last month saw Facebook CEO Mark Zuckerberg talk about his vision for a privacy-focused platform, which included the previously announced plans to integrate the messaging services of Instagram, WhatsApp, and Messenger. Facebook says these individual apps will remain, and Wong believes if chat is eventually integrated into the core app, the Messenger application will be used to serve a different market.
Facebook already owns WhatsApp, the world's most popular chat app with over 1.5 billion users worldwide. Messenger is in second place with 1.3 billion+ users.
Many people are likely to welcome the re-integration of a chat function directly into the Facebook app, which aligns with the company's new focus on using the platform primarily as a tool for private, encrypted conversations with friends and family.