The Association of Religion Data Archives (ARDA) only began to differentiate between Jewish ‘schools’ (probably more appropriate than denominations), in its 2010 Report.
In its 1990 Report, it found that there were 5,982,529 Jews in the United States, with 3,975 congregations. This constituted 2.4% of the total population.
In its 2000 Report, there were 6,141,325 Jews in the United States, with 3,727 congregations, constituting 2.1% of the total population. So… while there were almost 200,000 more Jews, around 200 synagogues closed, this is a clear sign that Jews as a total became less religious during this time.
In its 2010 Report, the ARDA differentiated between the Orthodox, Conservative, and Reform Jews. The numbers in that report were:
Orthodox Jews: 947,020 adherents, 1,932 congregations, 0.3% of the population
Conservative Jews: 501,776 adherents, 592 congregations, 0.16% of the population
Reform Jews: 766,352 adherents, 845 congregations, 0.25% of the population
It did also list the Reconstructionist Jews referred to by meltzerboy.
Reconstructionist Jews: 41,436 adherents, 95 congregations, 0.01% of the population
The only other relevant entries were two entries for Messianic Jews:
“Union of Messianic Jewish Congregations” had 65 congregations associated with it.
“Association of Messianic Congregations” had 12 congregations associated with it.
The total of the 4 Jewish groups there is: 2,256,584 Jews, 3,369 congregations, .72% of the population.
So, roughly 3.8 million people are no longer considered to be religious by the ARDA’s methodology. They only keep track of religoius bodies and their membership, congregations, etc. Those 3.8 million Jews are now lumped in to the 51.2% of the American population which is “unclaimed” to be the member of any religious body. So, they’re now part of 158 million Americans not listed with a religious body.
So, roughly 60% of Jews are not even members of a religious body. But, the members that are listed, may also not be practicing… so that number is probably higher. These numbers are only claimed members, and don’t reflect who practices.
Orthodox Jews are clearly the most religious of the Jews, but, since there are unlikely to be many “unaffiliated” Orthodox Jews, they do constitute around 1/6 of all Jews in America, if we’re still using the 6 million number, which includes all the cultural, secular, ethnic Jews, etc. who are again, not members of a religious body according to the ARDA.
If this were Yahoo answers, I’d request to be voted up as the best answer

This is the product of a lot of demographics and statstics number crunching from a poli-sci major :tiphat: