Beth Kowitt: Gates Foundation will close in 20 years.
She says it's the end of an era in billionaire philanthropy.
Gates and Warren Buffett introduced the Giving Pledge, she says.
But younger billionaires are less likely to commit to giving away their money, Kowit says.
The model of philanthropy that Gates represents is far from perfect, she writes.
A customized collection of grant news from foundations and the federal government from around the Web.
In the world of social enterprises, failure is a cringe-worthy moment nobody wants to talk about. But, social entrepreneurs can benefit from their failures.