With over 800 million monthly active users now - up from 500 million in November last year - Facebook Messenger became the fastest growing app of 2015, a study by global research firm Nielsen says.
Facebook Messenger was the second most popular iOS app behind Facebook with 10 billion photos being shared among users on it per month, it noted.
"From videos to music to apps, digital was at the forefront of consumers' (and marketers') minds this year," a Nielsen statement said.
"In 2015, Facebook again took the lead as the top smartphone app. The social networking app had more than 126 million average unique users each month, a growth of 8 per cent from last year," it added.
Reacting to the news, Facebook head of messaging products Dave Marcus said in a post: "The Messenger team's mission is to make Messenger the best place to communicate with all the people and businesses in the world."
"I'm happy to report that we've made a good step in that direction, and toward the end of 2015, we crossed the milestone of 800 million people using Messenger every month," he added.
YouTube came in second with over 97 million average unique users each month, followed by Facebook Messenger with more than 96 million average unique users each month.
"The apps with highest year-over-year change was Facebook Messenger with a 31 percent increase in users from 2014, and Apple Music, with a 26 per cent growth," Nielsen noted.
In the third quarter of 2015, a majority of US subscribers used Android (53 per cent) and iOS (43 percent) devices to access their apps.
Three percent of US smartphone owners used a handset that operated on a Windows phone, followed by 0.7 per cent on a BlackBerry.
The results are based on Nielsen's monthly survey of 30,000-plus mobile subscribers aged 13 and up in the US mobile owners.