He's not the source of the info, he's just the editor-in-chief of the publishing group that made it available to the world. If you want to make your Person of the Year somebody who actually had a shitload of influence (which, if I recall, is the sole qualifier for the title), give it to Bradley Manning. He's the one who leaked all them cables with his own welfare, freedom, and even life on the line (because he is an American citizen, a treason verdict isn't exactly a far-fetched outcome). And in return, his entire government is basically against him. Mike Huckabee is
openly calling for his execution, and doesn't care who hears. Manning's actions sparked an uproar in the international relations community. Even my mom, who is just a secretary, hates the man on a level akin to Orwell's Goldstein.
As for Mark Zuckerberg, I don't recall there ever being a point in time where the world at large gave two shits about his actions. He's just a dude who is really wealthy from an idea that, in it's most basic form, is not by any means original. It's a conceptually identical copy of Myspace. The only influence they have over world events is what the members are using it for.