Facebook announced a new app yesterday called Messenger. The app is available now on both Android and iOS and is a simple group messaging app. It doesn't work with Facebook Chat, but is rather a stand alone app that provides group messaging similar to iMessage, Kik, Google+ Huddle, etc. It doesn't provide read/delivered notifications yet, either.
If you recall, Facebook acquired group messaging service Beluga some time ago, so this is likely just the first of what we can expect from that purchase. Why Facebook chose to release two separate apps rather than tie the messaging service into their main Facebook app is unclear. You can download the app from the Android Market.
I have no interest in Facebook and will never use this app. But it's an interesting app nonetheless. I think it's clear that Facebook is trying to capture the growing messaging market as quickly as possible. Text messaging is on a decline, especially as more people get smartphones. It's only a matter of time before a superior service takes off as the SMS replacement, which is why we see so many services like this vying for the space. Facebook's solution isn't as elegant or well integrated as others, but it has the Facebook name attached to it, which is a huge advantage.
Ryan makes some good points, and I'm excited for something to take over for texts. I don't think this is it, though. It's arriving kind of late in the game and the slow start (lack of notifications, for example) doesn't impress me. I'm sure the Facebook brand will draw some people, but I don't see that being a huge advantage without being directly integrated into the site. Even those other more established messaging services seem to be a little slow catching on, so I predict this will lag behind.