Mark Zuckerberg announced his and wife Priscilla's second major charitable contribution to date -- a gift of 18 million Facebook shares to the Silicon Valley Community Foundation. The gift was announced, not surprisingly, by a post on Zuckerberg's timeline Tuesday evening. Facebook shares are currently valued at $27.71 each, so the 18 million share gift is valued at $499 million. The Facebook founder signed The Giving Pledge in 2010, a commitment to donate the majority of his wealth to charity during his lifetime. A few months prior, he gave a $100 million gift to the Newark City Public School System. That time around, however, he made the announcement on Oprah. "I'm really proud of the work we've done there, helping leaders like Governor Chris Christie and Mayor Cory Booker sign the most progressive teachers contract in our country, opening four new district high schools, 11 new charter schools and more," Zuckerberg wrote in his post. Read the full story at Mashable.