He's not selling the shares. He said they "will give 99% of our Facebook shares." If he sold appreciated stock and donated the proceeds, he'd still have to pay tax on the capital gains, and could only deduct the amount of the actual donation, which would be smaller after tax.

By donating stock directly, he still gets to deduct the full value of the stock without paying any tax, and the charity (or more likely, private foundation) gets to keep the full value without paying any tax. And it could take as long as it wanted to liquidate the stock and get the best price.