Are the numbers reliable? The United States, according to the data here has only 2.5 million out of the global population of 1.5 billion Muslims, which is based primarily on self-reporting via Census data. Similarly, China only records 21 million Muslims, where the figure might be two or three times as much. In any case, take a look for yourselves -- if there is any definition to where the Muslim world is, this is it! (p.s. if you are reading this in your email, you will have to go to the website as the formatting won't come through).
Interactive Data Table: World Muslim Population by Region and CountryFor information about how data for each country or territory were collected and analyzed, see Data Sources by Country. Sources include national censuses, demographic and health surveys, and other general population surveys and studies. Population figures for previous years have been projected forward to 2009 based on the assumption that the Muslim population of the country is growing at the same rate as the general population.
Data for countries marked with an asterisk (*) are drawn primarily from general population surveys, which have smaller sample sizes than demographic surveys and are not designed to measure the size of small minority populations. This may lead to undercounts of Muslims in countries where they represent a small minority of the population and overcounts where they represent the vast majority of the population. Those numbers, therefore, should be considered more approximate.