The Atlantic explains…
To make a call, all Facebook users have to do is tap the little “i” at the top right corner of the revamped [Messenger mobile] app, which leads to the screen pictured above at right. As long as the other caller has the Messenger app, U.S. users can now tap Free Call, and, well, it’s just like making a phone call, really — except that instead of ringing, a push notification pops up: “You have a call from Carly.”
In other words, it’s like Skype – except most people probably have a lot more Facebook contacts than Skype ones. And while the service is free, it may eat into your data plan if you’re not on a Wi-Fi network. For some people, data is more valuable than minutes. This would be less attractive to them.