Finally we get to the real story of the movie - why Eduardo is in court against his old friend. Facebook has grown, but they have trouble attracting investors. Sean Parker, allegedly the founder of Napster here (what happened to Shawn Fanning or have I got that totally wrong? Wasn't he the sole creator of Napster? Or was 'The Italian Job' right all along and he stole someone else's idea??) - anyway he finds out about the site and wants a meeting. Straight away, Eduardo doesn't like him, but Mark is drawn to him. Sean suggests calling it just "Facebook" and not "The Facebook", and tells them to move out West.
And so for the summer Eduardo takes an internship in New York while Mark follows Sean's advice. Apparently Eduardo left his post after a day and spent all his time travelling to investment meetings - but without success and his effort was wasted. Mark is living in a type of crazy frat-inspired house, finally belonging to a little club.
Meanwhile, while in England at a major rowing contest, the twins discover that Facebook is global. The Winklevosses decide to sue Mark for the theft of their idea.
Eduardo is tired that his calls are unanswered, and he feels marginalised - especially now that Sean seems to be making business decisions. Mark forgets to pick him up from the airport, and he calls round the house to find Sean acting like one of the team. Later, to get his own back, he freezes the business account to get some attention.
It works and Mark calls him - angry that he might have jeopardized the company. He is told to get to their new HQ. He arrives to be told that Sean helped secure additional $500,000 investment - and with lawyers he signs a contract for the new share amount. Things are rosy.
Soon, he is summoned back. He is presented with a new share certificate, that he refuses to sign. In the deposition it is revealed that with furter investment, more shares were needed to be generated. However Mark still kept his 51%, Dustin his 6 or 7%, Sean his 7%, the first investor his 6% ... and Eduardo's had been diluted from 34% to 0.03%. Mark blames Eduardo for not reading the fineprint.
At the HQ in the earlier timeframe, Eduardo confronts Mark and Sean. Sean offers him the $19,000 investment money back. Eduardo goes to hit him, making Sean flinch like a coward. He leaves. Embarrassed, and to relieve tension, Sean suggests to the office they throw a party that night to celebrate their first '1 million Facebook users' breakthrough moment.
The party ends up at Sean's house. Some of the interns bring drugs to the party. The cops show up, and Sean is arrested. Mark hates bad publicity, so Sean is fired from his position as President, but can still keep his shares.
It ends with Mark, alone on his laptop, in the deposition room. Karen from The Office enters to tell him they will be settling the case with Eduardo - basically because Mark has no personality, and any lawyer could make a jury turn against him - especially as there is a lot of unanswered issues such as did Mark inform the Harvard paper about Eduardo's chicken, did he plant drugs at the party and phone the cops etc. Mark accepts the advice.
He asks if she thinks he is a bad person. She tells him no, but it seems he is trying his best to be one. As it ends, Mark is on Facebook and invites Erica to be his friend - and then refreshes the status every few seconds to check if she has accepted yet.
We're told that Mark settled with the Winkelvoss twins and the other guy, and they won $70million (not bad for an unoriginal idea without a signed NDA!). He settled outside of court with Eduardo for an unknown amount, and Eduardo's name was added to the Facebook masthead so it says co-created by Eduardo Saverin. Facebook is now worth an estimated $25billion, making Mark Zuckerberg the youngest billionaire in history.