The Pew Forum on Religion and Public Life just recently released a report on their latest U.S. Religious Landscape Survey. Not surprisingly, the number of the respondents who identified themselves as Buddhist was pretty small, only 0.7%.
Here's a summary (in %):
Christian 78.4 (includes Mormons, Jehova's Witnesses)
Other Religions 4.7
Other World Religions <0.3
Other Faiths 1.2
Don’t know/Refused 0.8
But what is more interesting is that most Buddhists in the US are not immigrants who were born into Buddhism:
In sharp contrast to Islam and Hinduism, Buddhism in the U.S. is primarily made up of native-born adherents, whites and converts. Only one-in-three American Buddhists describe their race as Asian, while nearly three-in-four Buddhists say they are converts to Buddhism.The report showed that 75% of American Buddhists also have at least some college education or better.
Also interesting is that "Mainline Protestants, Mormons, Buddhists and Orthodox Christians also tend to have higher income levels, with pluralities of each of these groups making more than $50,000 per year."
Overall the sample size for the survey wasn't large enough to get an accurate count of adherents to individual Buddhist sects. Numbers for the categories counted, Theravada/Vipassana, Mahayana (Zen), Vajrayana (Tibetan), "Other Buddhist groups," were all < 0.3% with "Buddhist, not further specified" listed at 0.3%.
The full report is also available for those wishing to check out the minute details.