Nimbuzz Windows Phone app just got updated to v1.1.2 with lots of improvements and few new features. This messaging app now supports offline messaging for Facebook chat as well. We are yet to check how good it works, but you can give it a try.
-Get connected automatically to your friends already on Nimbuzz
-Get phonebook based friend suggestions, which you can accept or ignore.
-Phone number based registration introduced for quicker, safer password retrieval.
-Tap on any avatar to enlarge and view your contact’s full profile picture.
-Chat with your FB friends even if they are offline/Notifiable. Nimbuzz now allows you to send Push Notifications/offline messages to them.
-Overall performance Improvements (Connectivity, Design, Speed, Bug Fixes)
Download it here from Windows Phone Store for free.