The Messaging app has been replaced with Skype for Windows 8.1 and Windows RT 8.1.
With Skype, you can chat and make free Skype-to-Skype audio and video calls to your Skype and Messenger contacts. You can also use Skype side by side with two or more apps which makes it easier to switch to other things like browsing the web, listening to music, or playing games.
To move your Messenger contacts to Skype, sign in to Skype with your Microsoft account. To learn more about how to use Skype, see Chat, call, and video call on Skype.
You can chat with your Facebook friends with Skype for Windows desktop.
Skype for Windows desktop can only be installed on Windows 8.1 PCs, not on Windows RT 8.1 PCs.