There about 820,000 Orthodox Christians in the United States, of whom about 200,000 regularly attend church, according to the 2010 Census of Orthodox Churches in the United States. The number rises to 1,050,000 and 280,000 if the Oriental Orthodox Churches that are not in communion with Orthodoxy are included (these churches are sometimes called the “monophysite churches” because they reject the fourth Ecumenical Council). The Orthodox and Oriental Orthodox make up 0.34% of the American population.
The census, which got its data from parishes and not from official church bodies, counted only “adherents,” meaning those who had some connection with the life of the Church, which includes many who consider themselves Orthodox but don’t. It also distinguished adherents from regular attenders.
The census reports that the Orthodox Churches grew in the number of parishes from 2000 to 2010, from 2,036 to 2,370, but does not say anything about change in membership. Almost half the parishes belong to the Orthodox Church in America (551) or the Greek Orthodox Church (525), and the average average parish has 438 adherents (the Greeks having the highest average, with 908 adherents in each parish). The smallest are the Albanians (700 adherents in two parishes) and the Bulgarians (2,600 adherents in nine parishes).
California has the most Orthodox citizens (14.5% of the total) and the highest number of parishes (255), New York the second most (13.5% in 240 parishes).
As a percentage of the state’s population, Alaska has the most Orthodox (1.93%), followed by Massachusetts(0.93%), New Jersey, New York, and Rhode Island. The states with the lowest percentages are New Mexico (0.1%), North Dakota (0.2%), Mississippi and Hawaii (0.3%), and Kentucky, Idaho, and Vermont (0.5%).
A great many more details and all the tables are available here.
Thanks to Byzantine, TX for the lead.