That is simply not true, see: http://www.adherents.com/rel_USA.html#religions Showing that the number of self-identified adherents of Christianity (76.5% of US population) grew by 9 million (5% growth rate) between 1990 and 2001; for Islam (0.5% of US population) the growth was 0.6 million (109% growth rate). The numbers of agnostics (0.5% of US pop) declined by 16%.
Otoh, the number of Unitarian Universalists (0.3% of US pop) grew by 127 thousand (25% growth rate). However, the number of nonreligious/secular grew by 14.4 million (110% growth rate).
So what these numbers show is growing religious polarization of the US society - the worshippers of a dead man on a stick gained 9 million adherents while nonreligious increased by 14 million.
Wojtek