Android: Facebook's mobile apps are notorious for hogging battery life and resources. So, you'd probably love a stripped down Messenger app that was only 10MB, right? Well, it exists, but most countries can't use it yet. Facebook announced this version of its Messenger app for developing markets, including Kenya, Tunisia, Malaysia, Sri Lanka and Venezuela, with support for other markets arriving "in the coming months". For those in established markets like Australia, your best bet is to keep an eye on sites like APK Mirror for a third-party upload. Though, keep in mind, there's no guarantee the APK will turn up or that it will support your language.
While it's understandable that Facebook wouldn't want to offer multiple messaging apps at once (they're not Google, after all), a lightweight Messenger app is pretty high on users' wishlists. For now, the best bet to save your phone's resources is still the Facebook mobile site, but hopefully demand will prompt Facebook to roll this app out more broadly.