The Census Bureau has just released the American Community Survey numbers for 2010, using data calibrated to match with the 2010 census. At least, calibration is the best reason for why the ACS believes that New York went from 8,391,881 people in 2009 to 8,184,899 in 2010 (according to the new Factfinder). Because of such jarring discrepancies in results, people should under no circumstances directly compare numbers from the 2010 ACS with numbers from previous ACSes.
The best demographic survey in the US is still the 2009 ACS, which avoids the whopper claim that New York added more housing units than people at a time of skyrocketing rents, and should be used until it becomes completely outdated.
And even if 2010 census data is at all reliable, it’s still not directly comparable. Claims about absolute mode share or commute time are okay (the census after all only underestimated New York’s population by about 3%), but claims about change from 2009 are not. At best the 2010 ACS should be compared to the 2000 estimate base, and even that is strained – too much reliance on a census that doesn’t count everyone, insufficient reliance on years of rigorous statistical sampling.