Facebook is currently testing out its Messenger for Windows on Windows 7. It is a desktop client and at this moment, it is for a limited test group users. It provides access to Chat, the Ticker feed, and notifications.
At this moment, Skype, Yahoo! Messenger and Windows Live Messenger can be linked to Facebook. Once linked up with Facebook, you will also get notifications, chat and etc. It is similar to Facebook Messenger for Windows.
I guess that is why Facebook is testing out its own Desktop client to a limited group of users to gauge the interest. Anyway, there is no link to get this software. Instead, Facebook is kind enough to provide a link to its help page to let you understand more about this Messenger for Windows.