February 8, 2011; Source: MSNBC | Even though more than 50 billionaires announced last year that they would eventually give at least half of their wealth to charity as part of The Giving Pledge, few made big gifts in 2010, according to a survey conducted by The Chronicle of Philanthropy and Slate of the 50 most-generous donors in America.

Donors from this year’s top-50 list gave a combined $3.3 billion to charity – the smallest sum since The Chronicle began tracking the biggest donors in 2000. What’s more, only 17 of the 400 wealthiest Americans as ranked by Forbes magazine appeared on this year’s list.

At a time when overall giving is down and need is way up, many nonprofits have come to rely on wealthy donors to sustain operations. Leading the list was George Soros, the hedge-fund manager, who gave $332 million to his Open Society Foundations, which support democracy and human rights in more than 70 countries.

New York’s billionaire mayor Michael R. Bloomberg came in second this year. He divided $279.2 million among nearly 1,000 charities. Third was T. Denny Sanford, chairman and CEO of United National Corp.

For a full list of the donors and the charities they supported, visit The Chronicle and Slate. –Aaron Lester