Microsoft started sending out mass emails on Tuesday to its Messenger users, which number over 100 million, to announce the retiring of the Windows Live Messenger platform in favor of Skype as of March 15, 2013. As of March 13, all users will be moved completely to the Skype service, giving Windows Live Messenger just 66 more days to survive, at least for locations outside mainland China:

On 15th March 2013 we are retiring the existing Messenger service globally (except for mainland China where Messenger will continue to be available) and bringing the great features of Messenger and Skype together. Update to Skype and sign in using a Microsoft Account (same as your Messenger ID) and all your Messenger contacts will be at your fingertips. You’ll be able to instant message and video chat with them just like before, and also discover new ways of staying in touch with Skype on your mobile and tablet.

We’ve known about Microsoft’s intent to retire Messenger since September, but this is the first time an official date outside of “1st quarter 2013″ has been revealed by the company.

For those still using Windows Live Messenger, it’s highly recommended that you merge your account with your Skype service as soon as possible to ease the transition.

The email contains instructions for downloading and installing Skype. It also features this brief FAQ:

So, what’s happening between now and 15th March?
Messenger will continue to work as you know it today. If you are signed in with Messenger on your desktop** you will see a banner notification to upgrade. When you click on the banner, an installer window will open with the request to upgrade. This will take you through our installer flow to install Skype and automatically uninstall Messenger.

So, what’s happening after 15th March?
Messenger users on desktops** will not be able to sign in and will only be able to upgrade to Skype. If you attempt to sign in, a notification will appear, and if you continue, you will be taken through our installer flow to install Skype and automatically uninstall Messenger at the same time.

Can I update to Skype on my mobile?
Yes! Skype is available on iPhone, Android and soon on Windows Phone 8. We encourage you to download the latest Skype app on your mobile and then uninstall Messenger. You will be able to sign in to Skype on your iPhone, Android and Windows Phone 8 mobile apps with your Microsoft Account over the next few weeks. If you use another phone with Messenger on it, it will continue to work for a while.

* To get group video chat, you’ll need a Skype Premium subscription.
** Newer versions of Messenger will be able to receive the optional upgrade notifications. Older versions will not receive the notifications and you will have to download Skype manually.

We’ll keep you in the loop as the sunset date gets closer.