The FBI data captures only part of the total number of ex-Americans. Some people “relinquish” U.S. citizenship – for example, by taking citizenship in another country while intending to lose U.S. citizenship, then asking the State Department to document the loss. Relinquishing is more attractive for several reasons, not least because it’s free – a renunciation costs $450 US.As Cain points out, you can FOIA all you want, you won't get a full picture of the exit door. Cain compares this to the Canadian side of things:
Between 2008 and 2012, an average of 172 Canadians renounced their citizenship each year – typically because they want to run for office in a foreign country, join a foreign country’s military or become citizens of a country that doesn’t allow dual citizenship, Citizenship and Immigration spokesperson Mary Jago explained in an e-mail.As to "wanting to run for office in a foreign country": a likely rare but politically interesting sub-category. Canada is of course a much smaller country than the US so we can expect the number to be accordingly much more modest, still, here it is just about 5% of the documented US number, and therefore likely a tiny fraction of the whole US picture. I expect that picture to substantially expand again in 2014.