It seems Facebook Messenger is widely popular. According to Quartz who referred to a Sensor Core report, the app has accumulated more app downloads than the other Facebook apps combined, including the official app itself.
From the report, between 2012 and 2015, Facebook Messenger has been downloaded 646.6 million times, which is more than the official Facebook app (568 million). When you add the other Facebook apps (excluding Whatsapp and Instagram), they are still lower than the download count for Messenger (646.6 million vs 637.5 million).
The company has seen its potential where it has accumulated over 1 billion downloads on the Play Store & has a monthly user base of 700 million users. This is why they started by making a standalone web app for Messenger then went ahead and added video calling support on the platform. They also decided to create a platform to third party developers where they could develop apps and games that would be integrated in Messenger. They made it a standalone app where you can sign for it without necessarily having a Facebook account. Recently, there was a rumour where they’ll launch a virtual assistant for Messenger that will make it easy for people to buy stuff on the app.
From what I see, Facebook is building an ecosystem where users will be able to play games, video call and order stuff on Messenger, Facebook itself will be used for keeping up with updates from other people and brands, Instagram & Whatsapp is all the additional data. Its actually scary how much data the Facebook ecosystem has: They have your birthday, contacts, email address, geotag information from pictures, your preferences, your daily itinerary etc. A person can know quite a lot about you if they take a look at your profiles in the Facebook ecosystem and this is the stark future. Facebook Messenger will only get better and better from now on to acquire that user data social networking apps dream of.