"THE Social Network," you re understandably sick of hearing, is a brilliant movie about the Harvard upstart Mark Zuckerberg and the messy birth of his fabulous start-up, Facebook, circa 2004. From the noisy debate over its harsh portrait of Zuckerberg, you d think it s a documentary. It s not. Its genre is historical fiction - with a sardonic undertow. The director David Fincher and the screenwriter Aaron Sorkin are after bigger ironies than the riddle of Zuckerberg, a disconnected geek destined to spawn a virtual community of 500 million friends. You leave the movie with the sinking feeling that the democratic utopia breathlessly promised by Facebook and its Web brethren is already gone with the wind.