Facebook Messenger will have 105.2 million active users in the US this year. Four out of 10 mobile phone users will use it, making Messenger the leading over-the-top (OTT) mobile messaging app, as explored in a new eMarketer report, “Facebook Messenger: Strategies for Customer Service, Ecommerce, Content and Advertising” (eMarketer PRO customers only).SHARE32364312What was once just a feature within Facebook is now one of the world’s most popular mobile messaging services.Facebook introduced the standalone app in 2011, but it didn’t take off until 2014, when Facebook removed private messaging from the main Facebook app. Mobile users had to either download the Messenger app or use Facebook’s mobile site for messaging. (Desktop users can still send and receive messages from the Facebook website.)The move to split off messaging caused confusion and a fair amount of user outrage, but many ended up downloading the app. The number of people using the Messenger app grew from 500 million per month in November 2014 to 900 million per month in April 2016.In June 2016, Facebook began informing users it would soon strip messaging out of its mobile site. That will make the app the only way to send mobile messages on Facebook.“Right now, we have two of the top messaging apps in the world; 900 million people use Messenger every month, and 1 billion people use WhatsApp every month,” Facebook CEO Mark Zuckerberg said during the company’s Q1 2016 earnings conference call.There are several factors driving Messenger’s worldwide growth, as well as mobile messaging in general. eMarketer forecasts that 35.0% of mobile phone users worldwide will use mobile messaging apps this year, up from 25.2% in 2014. Nearly half (49.3%) of mobile users in North America and 43.4% of those in Western Europe will use messaging apps this year.In a February 2016 study of worldwide internet users ages 16 to 64 by GlobalWebIndex, Messenger was more likely to be used than WhatsApp. As of Q3 2015, Messenger was used monthly by 37% of respondents, compared with 33% who used WhatsApp. Both services were far ahead of the other messaging apps GlobalWebIndex studied, including Line, Snapchat and WeChat.