Facebook's Messenger app has surpassed 800 million users, the company says, making it the fastest-growing app of 2015, according to research firm Nielsen.
Messenger, which Facebook created as a standalone app in 2014, now has more active monthly users than rivals Snapchat and Viber, but still lags the 900 million using WhatsApp, also owned by Facebook.
WhatsApp was one of the first apps to let people send and receive free text messages on smartphones, bypassing network charges.
Messenger users can also exchange messages, but the app also offers free video calling and some business services.
"One of the things we have to work on this year is this perception or mindset that Messenger is only to speak with your Facebook friends," said Messenger head David Marcus in a phone interview.
Facebook plans to make money from ads on Messenger, but has not said when it will do so.
The world's biggest online social network, which has 1.55 billion users, makes money selling ads that appear on people's timelines.
Messenger has added features over the past few months, enabling users to sign up without a Facebook account, make payments, video calls and communicate directly with businesses.
It worked with ride-hailing app maker Uber Technologies in December so users could request a ride through Messenger.
Facebook has also begun testing a digital assistant, called M, that operates through Messenger and can make restaurant reservations and airline bookings and perform other tasks.
The service is available to only 10,000 people in the San Francisco Bay area, but Marcus said Facebook hopes to offer it to more users later this year.