I was kind of surprised to see this new application/feature of Facebook being offered to everyone. I mean, did you happen to notice the pop-up window in the bottom right-hand corner?
I thought it was interesting that Facebook would offer something like this considering that Facebook has an open protocol, jabber, that will let pretty much any instant messaging application interface with Facebook to be able to communicate with other members of Facebook.
One of the things that you notice about the messenger immediately is that it looks almost exactly like the smartphone application that everyone uses. I just wrote an article recently on how to set up Digsby with Facebook, using jabber. For the time being, Digsby is my favorite application for managing my instant messaging needs.