Facebook clearly hopes to use messenger to draw more people to sign up for the social network, where new user activity translates into higher profits. (Read More: Facebook Revenue Jumps 32% Amid Gains in Mobile.)
The service is first launching in five key emerging markets on Tuesday: India, Venezuela, South Africa, Indonesia, and Australia. The company then plans to quickly roll it out to the rest of the world.
Facebook doesn't currently make money from its messenger app, which includes free texting, group chat, and photo-sharing. Facebook has always offered it ad-free as an extra service to keep people connected with their Facebook friends, and within the social network's eco-system. (Read More: Facebook Seeks to Capture More Mobile Photos.)
Plus, it works across devices—users can message from mobile devices to the desktop, and vice versa. So the more people message on any Facebook platform, the higher the level of engagement, and the more potential ad revenue for the social network.
Facebook could easily start showing ads on top of messenger, though it hasn't announce plans to yet.
This is just the latest in Facebook's moves to make money on mobile, including faster mobile apps and the roll-out of ads—sponsored stories—into the News Feed. (Read More: What's Next for Facebook?)
The announcement will also have implications for mobile carriers. If people are texting for free using Facebook messenger, they don't have to pay their carrier per text.
However, the messages sent are subject to the data-use limits—and fees—imposed by the carriers.
Facebook said it's working with carriers to give subscribers discounted and free access to Facebook Messenger data, with deals launching in Indonesia, with Telomsel, and India, with Airtel, in the next week.
—By CNBC's Julia Boorstin; Follow her on Twitter: