1.2 billion people are using Facebook Messenger every month, up from the 1 billion Facebook reported in July 2016. Considering 200 million users in total would be a significant user base for most apps, that kind of growth over an 8-month period indicates Messenger is doing very well.
Facebook Messenger inherently has a boost in engagement not seen with other apps due to its connection to Facebook, the world's largest social network.
We increased engagement massively in terms of number of messages sent per active user. We had double-digit growth percentage-wise, - Davis Marcus, head of Facebook Messenger