Facebook already released a messenger app a while back for their Windows Users which allows quick chatting and notifications on the desktop. Unfortunately, there isn't any official Facebook messenger app for Mac users.

facebook-desktop

Matthew Cobb, a graphic designer based in London, UK have made a beautiful concept of how Facebook messenger should look on a Mac desktop. The concept is really beautiful and elegant looking which showcases the Facebook messenger design. The messenger is pinned in the left part of desktop and is inspired from the current Messenger for Windows.

I like the Interface of Chat Heads, which is inspired from the Facebook Home app. I just wish someone turn this concept into a real Mac app.

Facebook Messenger Desktop Concept on Behance