According to the United Nations, world population reached 7 Billion in the year 2011. But according to the World Population Clock website as of now (2017) we are 7.5 Billion people living in this earth, It shows a live statistics of world population Link.
There are about 5 Billion phone users all over the world by Statista, Out of 5 Billion 1.5 Billion is smartphone users by 2014. They are expecting that this is going to increase by 2.5 Billion by the end of 2019. As Statista says that whatsapp has 1.2 Billion active users(2017). By this we can say that about 90% of smartphones got installed whatsapp in it. In the year it has 700 million users which has jumped to 40% more in the year 2016. This is one of the best App in all over the worldwide. As we all know that whatsapp got acquired by Facebook.Inc in the year 2014 by paying huge money. Karan Girotra has published an article in Harvard Business Review, What we are thinking right now, whether it is worth of this massive deal hbr.org.
Other billion downloads in Playstore (Android)
Facebook, Facebook Messenger, Instagram.
Gmail (First App to reach Billion downloads), Youtube, Google Maps, Google Chrome, Google, Google Drive, Google +, Hangouts
If we observe the above apps only two companies reached billion downloads mark, Out of which 4 apps owned by Facebook and remaining apps are from Google. Most of them reached billion downloads in the year 2014-2015, except Instagram which was reached in the year 2016.
There is Ted Talk about How to build a Billion dollar app by George Berkowski.