Facebook has announced that the number of people using its Messenger app has surpassed 1 billion a month -- a major milestone for what is arguably the most important tool in the company's growth strategy. A growing number of people are using Messenger to communicate with friends, send money, plan events, play games and tell jokes, the company said, claiming that it's one of only a handful of apps worldwide with so many users. The announcement follows a series of upgrades to the app, which averaged 800 million users per month in 2015.