It's 2003 and Mark Zuckerberg (JESSE EISENBERG) is so obsessed with being invited to join one of Harvard's prestigious and elite clubs that he doesn't realize he's offended his girlfriend, Erica (ROONEY MARA). Then again, while a brilliant student, he isn't the most socially adroit person. Thus, when Erica dumps him, he drunkenly puts her down in his online blog and then impulsively creates a site where anyone can rate Harvard women based on their looks.
That spreads like viral wildfire and gets him in trouble for hacking into the school's computer system. It also draws the attention of twins Cameron Winklevoss (ARMIE HAMMER) and Tyler Winklevoss (ARMIE HAMMER) who, along with elite Harvard club member Divya Narendra (MAX MINGHELLA), want to hire Mark to program a Harvard dating website they've thought up.
Seeing the potential in that, Mark creates his own version that goes above and beyond just dating, and thus "The Facebook" social media site is formed, with financial backing from his friend and fellow student, Eduardo Saverin (ANDREW GARFIELD). That quickly becomes popular and then spreads to other Ivy League schools and beyond, not only drawing the interest of groupies such as Christy Ling (BRENDA SONG), but also Sean Parker (JUSTIN TIMBERLAKE), the creator of the file sharing network service Napster. He sees even greater potential in the site that he says should just be called Facebook, but Eduardo isn't happy to see his role in the quickly growing company being pushed aside.
With legal depositions getting underway regarding the Winklevoss twins and Eduardo separately suing Mark respectively for allegedly stealing their idea and wrongfully shortchanging his involvement, we watch in flashback as the social media site is created and then grows into the behemoth it's become today.