Actually, Paula, while Sorkin may not have CREATED Zuckerberg, he most certainly CRAFTED his character in a specific fashion — to spotlight his anti-social behavior. To wit: The young woman he broke up with in the opening scene and to whom he reaches out in the very last scene via Facebook? That never happened. Indeed, Zuckerberg’s long-time girlfriend from his days at Harvard became his wife. The break-up is a total fabrication, but it fits what Sorkin wanted to craft in the character.
Sorkin admitted as much in his Golden Globe Award acceptance speech, thanking Zuckerberg for allowing him to make a “version” of Zuckerberg (paraphrasing).
So to your point, yes, Zuckerberg DID exist as a real human being. But some key choices Sorkin made in crafting the character added to his already compelling nature.