The "new" web version of Messenger was actually launched a year ago, under the domain messenger.com. It has been looking like that from the first day.

Messenger.com was launched on April 8 2015. http://thenextweb.com/facebook/2015/04/08/facebook-launches-...

According to this, the web app of Chat Center was launched on April 13 2015. http://archimedeslabs.com/2015/04/chat-center-rebrands-and-m...