FBNotifier is a free Facebook notifier that notifies you of any new Facebook messages or notifications in your system tray. FBNotifier looks and works a lot like Gmail Notifier, and is a really good option to get Facebook alerts in system tray.
When you install Facebook Notifier, you provide it with your Facebook account information. It then keeps sitting in your system tray, and monitors Facebook for any new activity. If you get a new message or notification on Facebook, it shows an icon in system tray that shows you have a new notification. Icon also displays a count of number of unread messages.
To see your messages, you can just click on that icon, and see Facebook messages in system tray, without a need to open Facebook in your browser.