Parley.im is an amazing desktop client for Facebook.
Invisible Mode - See who is Online while appearing Offline.
Away Mode - Sends an Auto Message everytime someone messages you while you are busy.
Supports Friendlists - Be Online to some while being Offline or Away to others at the same time. And group Friends.
IM Style Facebook Chat - Bigger Interface and windows to chat.
Flashing Windows - Whenver a friend sends you a message. Don't miss any chats anymore.
Friends Online Notification - A small popup alert everytime a friend comes online.
Fast and Stable - More responsive and stable than chating in any browser. Also list of Friends Online/Offline is more accurate.