Apologies if this is a duplication. CAIR (Council on American-Islamic Relations) recently completed a survey last year regarding mosques in the US; this Mosque Study Project was part of a larger study titled Faith Communities Today. According to CAIR, there’s a margin of error of + or - 5% in their results.
It’s an interesting report - check it out if you have the time: http://www.cair-net.org/mosquereport