FIM is a simple Facebook chat client that does its job pretty well. It supports Live Tile notifications and toast notifications as well. The app lacked for Group chat support which is now added in this update.
Changes in the recent update:
* Added option for send button (instead of enter key)
* Added Group Chat and Thread Messaging support
* Bug Fixes
The app is ad supported, so you can download it from the marketplace for free or you can even buy a ad free version for $1.99. Windows Phone Mango users may not need this app since Facebook chat it built into the OS itself.