that is disputable. There is no denying Americans are giving. In fact, we lead the world in charities, followed closely by Canada, Myanmar and New Zealand, which are tied for 2nd. But Americans mostly give their time, volunteering and offering a helping hand. In financial terms, we are behind Myannmar, Malta, New Zealand, Canada, The Netherlands, UK, and a few others. But this was last year. On the last 5 years average, we still lag behind Malta, Thailand, UK, Australia, Canada and a few more.
https://www.cafonline.org/PDF/WorldGivingIndex2013_1374AWEB.pdf
But the above report does not differentiate domestic and international charities. When it comes to international donations that's more difficult to track individuals who sent money abroad from the United States may or may not be American citizens, and in some cases are dual citizenship, for example, American/Israeli nationalities. As far as I understand - and those who know more about this please feel free to interject - the IMF only tracks FDI, portfolio investment and gifts, in terms of international money remittance. They are not concerned with the nationalities of the people sending the money. That makes it difficult to establish the citizenship of those who are donating money.