The Microsoft Messenger team is sending out e-mail letting everyone know that, as of March 15, 2013, the company is retiring its messenger offering globally, except in mainland China where Messenger will continue to be available. Those who want to continue staying in touch will have to get used to Skype.
Messenger and Skype features will be unified and any holder of a Microsoft Account can use that to log in with Skype (something that has been available for a while now). Everything will function normally until March 15 when users, except those in China, will not be able to sign in and will only be able to upgrade to Skype.
Source: Microsoft (via e-mail)