Microsoft announced last November it would soon retire its instant messaging client in favor Skype, which Microsoft acquired in October 2011. Today it began informing Messenger users that the service would go dark for the majority of users on March 15.

Hello,
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.
Yours sincerely,
The Messenger Team
Skype officials said in late October that Skype would most likely replace Messenger someday but had declined to provide a public timetable. A blog post in November indicated the switchover would occur during the first quarter of 2013.

Skype began testing new Windows and Mac beta releases in October that let users sign in to Skype using their Windows Live ID, allowing them to send and receive instant messages and see the presence information of those using Live Messenger, Xbox, Hotmail or Outlook.com.

After migrating from Messenger to Skype, users will be able to use Skype's instant messaging, as well as its video calling, landline calling, screen sharing, and video calling on mobile phones.

Yahoo News