Facebook released a lite version of its Messenger application near the end of last year. As with most Lite apps, it's designed to be used in growing markets, where cellular data may be a limited resource. It's less than 10MB, works on Android 2.3 or higher, and has all of the essential features of the full app.
Messenger Lite has now passed 50 million downloads on the Play Store. For context, the Facebook Lite app is somewhere between 500 million and 1 billion downloads, and it was released at the start of 2015. It's important to note that Google counts pre-installed apps as downloads on the Play Store, so if a carrier or device came with Messenger Lite, it could be skewing the numbers a bit.
If you live in one of the countries where Messenger Lite is supported, you can download it from the Play Store. Otherwise, you can get it from APKMirror.