I see the confusion. Exercising stock options means buying, not selling, and his tax bill will be from buying shares of stock. Put more simply, he'll be acquiring many new shares of facebook stock when it IPOs, but at a cheaper price (6 cents a share!) -- receiving this new stock will be treated as one-time taxable income. They must be non-statutory options, which I guess would make sense since they were granted pre-IPO.
If he sells them later, he'll have to pay taxes on them again, unless they fails to gain value after the IPO.
edit: You are correct about one thing, though -- turns out Zuckerberg himself will be selling some shares during the IPO as well (about 30 million out of the ~400 million being offered). I didn't know that. The article you linked explains why; he's about to buy 120,000,000 shares at 6 cents a share! That's quite a tax bill! So he's selling some shares he already owns so he can buy back shares at a cheaper price. Tricky.