Here's Facebook official description of the new software (typo corrected):
Messenger for Windows is a new, trial application that lets you use Facebook without being on www.facebook.com. While you surf the web or use other applications on your computer, you can:
Chat and message with your friends on Facebook
See the latest updates from your friends in ticker
Get quick notifications about what's going on
We're testing out a first version of the app with a small group of people. During this trial period, we plan on rolling out changes to the app and expect outages and periods of instability as we make improvements. Note: Messenger for Windows will automatically install updates.
Facebook is looking to gauge whether users are interested in using the social network's real-time features without having to keep a browser window open. The social networking giant is confident enough that it already plans to release a final version: "If you're not in the test group, you'll be able to get the app once we roll it out to everyone."
Facebook is clearly hoping that users will leave Messenger for Windows on throughout the day, probably since most Facebook addicts already do so with their browser. Still, the advantage of a desktop client is that it can use the operating system's notification system to alert the user: a browser can't do that, especially if you have Facebook open in an inactive tab.
Users who can get the new beta client are being randomly selected to join the tester group; they are being notified via home page prompts (there is no public download link), according to TechCrunch. The application was developed entirely by Facebook and does not constitute a new partnership with Microsoft, which is a big investor in Facebook.
Speaking of Microsoft, the software giant's Facebook-integrated Windows Live Messenger desktop client already lets you use Facebook Chat to communicate with your Facebook friends. It is currently the top app on the Facebook Platform when it comes to daily active users (18.2 million), and also has 21.5 million monthly active users, according to AppData.
It's not clear if Facebook will offer support for anything before Windows 7, or if it will simply move on towards Windows 8. Clients for Mac and Linux will likely only be released if the Windows 7 version proves very popular.