The Foreign Account Tax Compliance Act (FATCA) requires United States persons (definition of 'US persons' is vert wide), inclusing all individuals who live outside the United States, to report their financial accounts held outside of the US. From July 1st 2014, Passive Foreign Investment Companies (Banks, Insurance Companies, Investment Funds etc.) are obliged to report all their American clients with holdings over $50,000 to the IRS.

Only 7% of the 7 million American expatriates file a tax return. Source: Jan 28th, 2015, AmCham Seminar on Foreign Account Tax Compliance Act.