Except for China, for some reason, where the service will continue to be available. Microsoft hopes users move over to Skype, since they acquired it way back in 2011. Messenger and Skype features will be unified, and if you have a Microsoft account, you can use it to log into Skype as well. It’s also worth noting that your contacts will come with you when you upgrade to Skype.
So basically, if for some reason you still use MSN messenger, it’ll work until March 15th, after which it’s dunzo and you’ll need to switch to Skype. Microsoft is also making Skype an integral part of their ecosystem, from a focus on Windows Phone, Windows 8 and even Xbox.