It is one of those unofficial rumors that both make sense and don't. Based on a blurry photo, it seems that Facebook is working on a dedicated Mac app for its Messenger chat service, independent of the Facebook instance that run inside your browser. If it's true and if the app ever sees the light of day in public, it would be the first time Facebook would provide a separate chatting experience on the desktop, something that it has already done for the mobile counterpart.
Facebook has had a rather ambiguous relationship with desktops. Its only official desktop chat app was simply launched as damage control over an unofficial leak. Eventually, it was laid to rest. To this date and with the exception of the Windows 10 universal app, Facebook delivers its experience, both its news feeds as well as chat, through a web browser. For some, that might be enough. For others, it's an unnecessarily burdensome requirement.
To some extent, it makes sense. Facebook's main business is drive traffic to its news feeds and pages. Having Messenger tied to a web browser helps that by not letting users stray too far away. But separating Messenger from the main Facebook page also makes sense, especially if it wants to outdo its rivals. After all, it already has a dedicated web page for it and has divorced the two on mobile.
Facebook might still be weighing the pros and cons of that move, which is probably why this seemingly working app hasn't been launched yet. Aside from being an internal test that might never be launched at all. But while Facebook is taking its sweet time, third party app developers will continue enticing users with their unofficial wares. And Messengers rivals, like Hangouts, Viber, and Line, will do similarly with their desktop apps or browser extensions.