Muslimpopulation.com gets its information from an authentic source which is religouspopulation.com. this is the article by religiouspopulation.com and the research they present is done by Carniege Edowment of International Peace, thus we know this is accurate statistics.
"To generate a consistent database which would enable one understand the influence and trends of mankind within the premises of major religious faiths and practices, research has been carried out by utilizing multiple sources like Population Reference Bureau, CIA Fact Sheet, WIKIPEDIA , Pew Forum, Encyclopedia of the Nations, Holt Rinehart and Winston, BBC and other relevant databases.
The study establishes that the Muslim population is increasing steadily due to higher birth rate as well as conversion, while adherence towards other faiths are either dropping or remained static due to leaving the faiths or lesser growth rate. For example, the Christian population in the west & America(North & South) is dropping due to lesser growth rate through conversion or birth rate as compared to Muslim population and at the same time renouncing the faith system altogether. Christian population is in increase in many of Asian and African countries by both birth rate and conversion. Current data displays that in 2016 Christian population are about 2.07 billion and while Muslims are 2.14 billion.
We calculated on population data sheet of 2016 multiplying the growth rate such as Muslim 1.84% and Christianity 1.13% as found correctly from Carnegie Endowment for International Peace."
http://www.religiouspopulation.com