9to5Mac has screenshots of an early beta version of Facebook Messenger for iPad. The app features one-to-one messaging, group chatting, push notifications, the ability to see friend locations on a map, and so on. Facebook is eventually planning make a Facebook Messenger for iOS app that works on the iPod touch, iPhone, and iPad. That won't happen for a while though, given that the phone version is already getting other features, like video calling.
Facebook currently struggles to support its iOS app across all three devices. For example, Facebook for iOS version 4.1.1 was released last month. While the new version added support for the new iPad's Retina screen, it didn't really bring the iPad side of things up to speed with the iPhone side of things.
The previous release (version 4.0) added Facebook Timeline support for the iPhone but not the iPad. Facebook still hasn't given a date for that feature. Apple's tablet was rumored to be getting Timeline in January 2012 but obviously that didn’t happen.
Prioritizing Retina support over Timeline support makes sense, although it must be annoying having to port the old Facebook profile to the higher resolution screen only to have to do it again with the new Facebook Timeline design. I think Facebook is going to try to time the new release with whenever it switches everyone over to Timeline.