Nimbuzz for Windows Phone now supports offline messages/push notifications.

Nimbuzz is a multi-platform messaging app that supports multiple messaging services from Facebook Chat to GTalk and Yahoo Messenger. Besides chat services, Nimbuzz lets you also share pictures and videos with your friends and all that at no cost, except your data connection of course. The platform boasts 150+ million users. Among the updates, there is a feature that lets users send offline messages/push notifications to  facebook friends.

Check out the full list of new features:

-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)

Source: Windows Phone Store