Pew Research Center estimates that there were about 3.3 million Muslims of all ages living in the United States in 2015. This means that Muslims made up about 1% of the total U.S. population (about 322 million people in 2015), and we estimate that that share will double by 2050.At the moment, the U.S. population is overwhelmingly Christian or "no religion" (most of whom are probably "ethnic" Christians--born into a family that is/was nominally Christian). Jews are second on the list, albeit accounting for only 1.8% of the population, Muslims are third at 1% and Hindus are third at 0.7%.
By 2050 the current Pew projections suggest that Muslims will have overtaken Jews as a percentage of the population. This is because of expected continued immigration (Pew estimates that Muslims represent about 10% of legal immigrants but only a negligible fraction of undocumented immigrants) and because Muslims tend to have more children than the U.S. average. Furthermore, since the Muslim population is relatively recent, it is comprised of a fairly high percentage of young people, so their reproduction will contribute disproportionately to population growth.