UPDATE: Starting today, the new Messenger experience will be available more broadly to people using the app on both iOS and Android.
As more and more people use Facebook to connect with their friends on mobile, we are focused on building the best mobile products. Facebook messaging started as a desktop chat experience, but when it comes to messaging on mobile, people want something faster and lighterweight.
This is different than Home, which was a full mobile phone Launcher. This is a messaging app to rival the likes of LINE, WhatsApp, and the like. I get the feeling they're targeting Google Hangouts, which in Android 4.4 are taking over the phone's SMS messaging app to become the go-to messaging app on your phone and PC.