As you may have heard, Facebook just released a standalone Messenger app in the US (not yet released in Australia). It's their effort to invade your phone with a messaging platform that everyone has access to (because 'everyone' has a Facebook). And even if you don't use Facebook, you can use it to SMS!
What's it do?
Well, theoretically it can become your main messaging app. Think of it as a cross platform messaging service that enables you to easily talk to people on any platform — Android, iPhone, Facebook, SMS and everything in between. It's not dissimilar to Kik or WhatsApp in this aspect. Like Sam said, If your friends are on Facebook, they'll get your message in the Messages (or on their Android or iPhone). If they're not on Facebook, it'll be re-directed as a SMS message (they'll get a message from a random number saying CASEY CHAN SAYS ___ ). The only problem is that your incoming SMSes aren't re-routed through the app so it's not the be all, end all.
Why do we like it?
It's weird to think that this can be a standalone app apart from the clusterzuck that Facebook can be but, well, it really can. It's as good as GroupMe or Kik or WhatsApp in the sense that you're chatting with friends without racking up SMS charges but it's better than those services because most people have Facebook accounts and don't need to sign up for a new service. It's easy to add buddies to chat (through their Facebook account or phone number), almost scary easy to attach your location to each message and less complicated than GroupMe to start a group chat.
Even if you hate Facebook, you don't hate your friends right? The Facebook-ness of the app is invisible enough that even the most anti-Facebook guy will have to admit this is actually decent. Seriously, strip the Facebook away and what you get is a messaging app that your friends have already signed up for. When it becomes available in Australia, you can download the app from the links below.