The Census American Communities Survey estimated Detroit at 777,493 in 2008. An overestimate for sure, but closer than the alternate 900,000+ number people now remember. The corrected official number after the 2010 census was 756,383 in 2008. Here's an article from Crain's talking about the two different methodologies the Census Bureau uses.
http://www.crainsdetroit.com/article...lt-to-estimate
ACS data for 2015 isn't available yet, but the ACS figure for 2014 was 680,281 while the Census Population Estimates Program [[the "official" data) had the figure at 680,223 for 2014. So we don't see anything like the kind of discrepancy that was apparent in the late 2000's.