Last night Facebook announced the release of a new feature that enables developers to send live messages to users of their application. As far as I can tell, this does not mean that developers can message users at any time. Instead they can send these messages strictly when a user visits a canvas page or views a profile box.
There is documentation of the new feature on the developer wiki for those that are interested in learning more. The feature is perfect for multiplayer games since the application can create an instant chat between two users of the game even if those users are not currently friends. While games are not the only instance in which LiveMessage can be used, it is the best example.
Theoretically you could have a live assistant interacting with users visiting your application. This is great for future ecommerce applications that are launched whenever Facebook decides to launch their e-commerce system. While I haven’t seen the new service used in practice I’m guessing that we’ll see a lot of applications begin to take advantage of this new feature.