As you already have known Facebook app chat isn't something new, there's some FB chat apps you can use without you no need to connect to your Facebook account in your browser. There are some popular IM (Instant Messenger) clients such as: Yahoo messenger, Live Messenger, Adium, and more that support with Facebook chat intergration (It's because Facebook uses XMPP protocol to supports those IM).
Althought there isn't something new with Facebook chat messenger if compared with another chat messenger services, however according to Facebook Help Centre, Facebook promises to add some features such as: to enable chat with a few users, video calling, and more.
This is indicated that Facebook seriously to improve the chat serices features in the future that connecting between Facebook users from PC to mobile, after in earlier they have launched Facebook App Messenger for iPhone and Android.
This Facebook apps Windows Messenger is only available only for Windows 7 OS platform, and at the time this post is published the news was revealed that only certain people can try it with a notification on-screen appearance FB home page.