As is usual operating practice in the software business if it works well change it as Microsoft is done here. They purchased Skype a few years back and Skype has been used for many years as a telecommunications product.
Well since it supports chat, why should we have two chat products so they killed off Microsoft Messenger and they're forcing you to use Skype as your messenger product if you want to communicate with other people who used to communicate with using Microsoft Messenger.
So now you'll need a Skype account to communicate with those same people. Luckily you can use the same credentials you used for Microsoft Messenger once you've installed Skype. Of course once you have Skype installed you can also make telephone calls. And they're very high-quality phone calls to other people using Skype, or to land lines, or to mobile phones. Check out Skype at Microsoft website.