Facebook Messenger is one of the most popular messaging apps in the world right now, behind another Facebook owned property, WhatsApp. Today, David Marcus, the head of Facebook Messenger's team, announced on Facebook (where else would he announce this?) that there are now 1.2 billion people using Facebook Messenger every single month. Which is a pretty big number, and is very close to the number of monthly active users for Facebook itself - who has around 1.23 billion.
While many may feel that Facebook Messenger's active user numbers are a bit inflated, since Facebook does force users to install the app if they want to send messages, it doesn't mean that users have to use it, but they do. Unlike WhatsApp, which isn't forced to be downloaded or used, and is still a larger messaging service. Facebook Messenger has greatly evolved over the years, going from a simple messaging service, to basically being a social network that can also handle all of your SMS messages, putting your messages from multiple places all into one app (of course that only works on Android, since Apple doesn't allow users to use a third-party service for SMS). Facebook Messenger recently added Stories to the app, basically taking Snapchat and bringing it into their messaging service. There is also the ability to call people over Facebook Messenger, which means you can now call people in other countries without having to pay absurd roaming fees, it's all for free.
Facebook Messenger is also a big deal for companies, with Chatbots being part of the system now. Companies are able to do customer service over Facebook instead of forcing users to call them or use their own chat client. It's a simpler way, especially since most people follow companies they use on Facebook already. Facebook Messenger has a bright future ahead of it, and it likely won't be too long before we see it eclipse 1.5 or even 2 billion monthly active users. Remember these are the number of users actually using Facebook Messenger at least once a month, as opposed to the number of people that have installed the app at some point.