I got my number from adherents.com - here is a bit more depth:
Islam. In recent years Muslim leaders in the United States have optimistically estimated that there were approximately 6.5 million Muslims in the country (Aly Abuzaakouk, American Muslim Council, 1999). In 1998 a Pakistani newspaper even reported that there were 12 million Muslims in the United States (4.2% of the total population)! After the events of September 11, 2001, many newspaper accounts included an estimate of 8 million American Muslims. This would equate to 3% of the U.S. population, or roughly 1 in every 33 people in the country. No comparable figure has ever been confirmed by independent research similar to the Kosmin or Glenmary studies, or the Gallup, Harris, Pew, Barna polls. Currently, surveys consistently report less than 1% of people surveyed identify themselves as Muslims. Muslim community leaders say that many American Muslims are relatively recent immigrants who either do not have telephone service, do not participate in surveys or are afraid to identify themselves as Muslims for fear of anti-Muslim discrimination. Researchers generally agree that the estimate of 300,000 Muslims in the Kosmin study (1990) and Kosmin's adjusted estimate (to 500,000) are too small to reflect current (year 2005) numbers of American Muslims. In 2004 the National Study of Youth and Religion conducted by the University of North Carolina at Chapel Hill (sample size: 3,370 teens nationwide) found that less than one half of one percent (0.5%) of American teens were Muslim, a proportion right in line with the adult Muslim population, based on other studies. Tom W. Smith of the National Opinion Research Center (NORC) at the University of Chicago is a nationally recognized expert in survey research specializing in the study of social change and survey methodology. Smith published "Estimating the Muslim Population in the United States" in 2001. This is probably the most thorough academic study of this topic in recent years. This study concluded: "The best, adjusted, survey-based estimates put the adult Muslim population in 2000 at 0.67 percent or 1,401,000, and the total Muslim population at 1,886,000. Even if high-side estimates based on local surveys, figures from mosques, and ancestry and immigration statistics are given more weight than the survey-based numbers, it is hard to accept estimates that Muslims are greater than 1 percent of the population (2,090,000 adults or 2,814,000 total)."
Interesting discussion. Let's ask ourselves a few logical questions.
Australia has a population of 20 million. Our muslim population is around 350,000. We have a regular census, on which we are asked to ID our religion. (It's not compulsory to answer.)
Your population is around 300 hundred millions.
How would it be possible that you only have say, four times as many muslims in your country as we do?
It just doesn't make sense to me. It's not logical. Australia is only settled and fertile around the coastline. Its Centre is mostly desert. Your continent is filled with towns and cities! (In Australia, ostriches, wild camels, wild donkeys and kangaroos populate more territory than we do.)
I think, if you sat down with a sharp pencil, and started with your industrial cities, one at a time, beginning with Dearborn...you would get to 1.5 million in a few minutes!
I could be wrong...but hey, there are around four hundred of them in ICELAND!