Today I finished updating my expatriation graph back to 1997, the first full year that expatriation data was published in the Federal Register.
After looking at the data, I was quite surprised to see the big spike in 1997, with 1,812 expatriations. The second quarter of 1997 alone had 1,205 expatriations. In looking at the names listed in the federal register for the second quarter of 1997, I noticed many Asian names. Then it occurred to me that Hong Kong reverted back to China on July 1, 1997 (right after the second quarter of 1997).
After doing a little research, for reasons that I don't fully understand (but likely having something to do with China not recognizing dual citizenship), it appears many Hong Kong residents renounced their U.S. citizenship just prior to the turnover of Hong Kong to China. These Hong Kong expatriations triggered the 1997 spike in U.S. expatriations.
Thus, last year (2011) did not have highest number of expatriates in U.S. history, but perhaps 1997 did. Further, with the trend apparent in the graph, I wouldn't be surprised if the number of expatriates in 2012 outstrips even the number in 1997.
The updated chart is shown below.