It's not clear if its recently acquired group messaging firm, Beluga, had anything to do with it or not, but Facebook has branched out further into the increasingly crowded messaging space today with the announcement of a separate Facebook Messenger app. Available for iPhone and Android, the app lets you send messages directly to individual friends or groups, which they'll of course receive instantly (a la BBM, Huddle for Google+, and Apple's forthcoming iMessage). You're also able to share your location if you choose, and attach photos to your messages, but the app is otherwise streamlined and barebones -- which Facebook says is just the point. You can grab it from the App Store or Android Market now via the links below.
Update: As pointed out by Mark Levin in comments, it turns out the Beluga team did indeed have a hand in the development of the app.