The survey is in no way a definitive set of data. First off, On Device surveyed just 3,759 Android and iOS smartphone owners across US, Brazil, South Africa, Indonesia, and China, meaning no Windows Phone users, and certainly missing quite a lot of regions including the entire European continent. Additionally, the question that was used to determine which app was in the lead was, "Which social messaging app do you use at least once a week?" This obviously doesn't take into account frequency of use or number of messages processed.
44% of respondents said they used WhatsApp at least once per week compared to 35% who used Facebook and 28% who used WeChat, which has a very big user base in China. BlackBerry Messenger gathered 17% of users built mostly on larger usage in Indonesia and South Africa. Social messaging is by far the dominant way of communication these days with 63% respondents who used social messaging service saying they used it least 10 times a day.