According to a notice by the IRS published today on US Federal Register 576 Americans living overseas gave up their passports in the second quarter of 2014, making it 1577 in the first half of the year.
The drop of US citizenship is partly triggered the US Program for Swiss banks started in August last year and the Foreign Account Tax Compliance Act (FATCA) in place since July 1, 2014. Swiss banks have to hand over data on their US business in order to avoid prosecution for helping tax evaders.
It is worth remembering that US citizen have to pay US taxes wherever they reside. In this context, US citizens with a net worth over $2m and an average income tax of at least $157k over the previous five years must pay an exit tax on unrealized capital gains when they give up their passports.