Cross posted from USxCanada InfoShop
A search for the data that underlies “estimates” of the numbers of U.S. citizens resident outside the United States does not lead far. A total of 6.35 million, excluding military personnel, is one handy number.
A map  provided as part of the information on the web site of American Citizens Abroad (ACA)  appears to be as good a piece of information as any that can be obtained. And even that map data is contradictory (see note following citation).
ACA and Association of Americans Resident Overseas (AARO) pass along figures said to be provided by the U.S. State Department. The only level of detail is for seven regions that cover the entire world. These organizations have been hacking away valiantly for years at this data situation. It seems peculiar that a direct State Department source for this same data cannot easily be found.
Smith  cites older State Department estimates and says that “security concerns” prevent release of more current figures. The Johnson map  likewise contains a note saying that “security concerns” prevent access to any level of detail more fine-grained than the seven large regions.
In reality, “security” concerns seem to equate to embarrassment concerns. The same embarrassment circumstance may also apply to another tarpit of mushy data, the numbers for renunciations of US citizenship. In both cases, lowball might look better for the United States under present policies.
Perhaps the most solid general indicator of numbers is recent ten-year data that indicates “503,585 consular reports of birth abroad (CRBA) were made and passports issued as a result by U.S. embassies between 2000 and 2009” . Factor in births not reported. Consider likely ratios of adults to children. Adjust for possible divergence from norm among adult population. Etc.
There is a history of how the U.S. census has and (mostly) has not dealt with extraterritorial citizens —  and . Any further pursuit of that topic would be a digression from the focus of this short essay.
The ironies of this situation astound. The United States clearly has no good data on citizens who reside outside its boundaries — nor much will to use the U.S. census to develop such data.
Nevertheless, whoever those yet unknown citizens may be, and wherever they may live, the United States seems eager to pursue them, to try to extract resources from them, to give them almost nothing, and to subject them to neverending uncertainties. Especially the ongoing uncertainty that surrounds whatever new measures it may unilaterally decide to unleash on them next.
One unhappy likelihood is that the United States will only deepen the morass that already confronts persons who seek to shed that toxic US citizenship.
In 2012, enjoying liberty translates into declaring independence from the United States?
* * *
 How are we counted? American Citizens Abroad (17 Oct 2011)
 Don L. Johnson. American citizens living aborad [map based on U.S. State Dept. estimates 2011] Association of Americans Resident Overseas (4/08/11)
[Note: Map states total of 6.32 million, yet figures for seven regions add up to 6.35 million]
 Claire M. Smith. These are our numbers: civilian Americans overseas and voter turnout Overseas Vote Foundation (Aug 2010) 13 p.
 Claire M. Smith. Defining the universe: the problem of counting UOCAVA voters Overseas Vote Foundation (May 2009) 8 p.
 Issues of counting Americans overseas in future censuses. U.S. Census Bureau (27 Sept 2001) iii, 17 p.