Although the app leaked last year, Facebook has made their Messenger software available officially today.
The app will roll out to all users over the coming weeks.

There will be conversation tabs, navigation buttons similar to the touch and mobile versions of the site, pop-up notifications and even News Tickers. The app is exactly like the leaked version from December but Facebook says that version was just a "test."

Furthermore, the company says a Mac version is coming soon, but did not provide a time frame.