Messenger Companion is a browser plugin for IE7 or IE8 which lets you quickly discover what your friends have shared online, when you visit a website, MC will let you know any new shared links on that site by subtly flashing in top right corner of your browser window. It lets you view links your friends are sharing, comment on them, and even share something fun you've come across. And if you've connected your social networks to Windows Live, MC works across all of them.
If you want to check out all recent links shared on a site, all you need to do is use MC button in IE. When you join in a conversation, your comment gets posted back to social network your friend used to share link. When you share link through MC, it'll update your status in Messenger, and all your friends in Messenger will get an update from you about the link you shared.