Facebook Messenger has 1.3bn Monthly Active Users

"Meanwhile, Facebook Messenger as a whole continues its ascent, entrenching itself as the top cross-OS messaging app in the West. Messenger now has 1.3 billion monthly users, up from 1.2 billion in April and 1 billion in July 2016. That’s the same count as Facebook’s other chat product, WhatsApp.
Messenger’s growth rate has slowed slightly over the years. It took just six months to go from 800 million to 1 billion, and nine more months to get 1.2 billion, and five months to add the last 100 million. That could signal that Messenger is beginning to hit saturation in some core markets."

Source:  Techcrunch, 14th Sept 2017