SAN FRANCISCO (AP) -- Facebook CEO Mark Zuckerberg says he is donating nearly $500 million in stock to a Silicon Valley charity to focus on health and education issues.

The gift of 18 million Facebook shares is valued at $498.8 million based on their closing stock price on Tuesday, when Zuckerberg, 28, made the donation. The beneficiary is the Silicon Valley Community Foundation, a nonprofit that works with donors to allocate their gifts.

This is Zuckerberg's largest donation to date. He pledged $100 million in Facebook stock to Newark, New Jersey, public schools in 2010, before his company went public earlier this year.

Later in 2010, he joined Giving Pledge, an effort led by Microsoft founder Bill Gates and investor Warren Buffett to get the country's richest people to donate most of their wealth.