Two years ago Facebook Messenger’s capabilities were so limited that you could not do simple things like send friends an animated GIF, as you could with many other messaging services.

Since mid-2014, Facebook has been playing a furious game of catch-up with its Messenger service. That June, Facebook’s chief executive Mark Zuckerberg hired a PayPal executive, David Marcus, to take over Messenger and build it into a world-class competitor. The company has added a string of features to the service, including letting people send money to friends through the app, pull up a voice or video call, or order a private car.

Facebook recently said it would also begin testing “secret conversations” inside Messenger, a feature that offers end-to-end encryption on some messages to be read only on the two mobile devices that users are communicating with. While it stops short of the full encryption that other messaging services like WhatsApp have adopted, it gives Messenger a heightened mode of security that Facebook hopes will attract global audiences to download the app.

“The fact that we have 1.65 billion people on Facebook already makes Messenger the best live, self-updating address book in the world,” Mr. Marcus said in an interview. “Because of the scale of our network, I feel like we really have a shot at this.”

According to industry estimates, roughly half of all American smartphone owners use the Messenger app and Facebook is aiming to capture greater numbers in international markets. Most of Messenger’s success has been in English-speaking areas like North America and much of Europe as well as Australia and large parts of Southeast Asia.