1 Sean Parker was shown Facebook by his Girlfriend
Sean Parker saw his girlfriend using Facebook , which was at a very early stage, and he liked it.
2 Sean emailed Mark Zuckerberg
Sean Parker decided to email the one behind that social network. He emailed Mark Zuckerberg asking to meet him.
3 Mark was interested in the meeting
Sean Parker was famous for being part of Napster which became a very famous software before being forced to shut down by law. Mark Zuckerberg was looking up to the meeting.
4 Sean Flew to New York
Sean Parker Flew to New York to meet Mark Zuckerberg. The meeting went well and Mark was impressed. It was Sean who suggested calling the website Facebook.com instead of thefacebook.com.
5 They met again in Palo Alto
Sean Parker and Zuckerberg met again by coincidence in Palo Alto. Mark had rented a house to run Facebook from it while Sean was living with his new girlfriend. (See How Mark Zuckerberg spends his day?)
6 Mark invited Sean to move in
Mark Zukerberg invited Sean Parker to move in and help with Facebook. Sean moved to the Facebook's summer house and started helping Mark with the project.
7 Parker helped Facebook
Parker helped Facebook get proper financing. His advice to Mark Zuckerberg was smart and he was the one telling Mark Zuckerberg that Facebook will be really big.
8 Parker joined Facebook
Parker joined Facebook. He became the president of Facebook and the shares he got made him later worth few Billions. (See How did Mark Zuckerberg come up with the idea of Facebook?)