The Social Network/WMG

The film's factual misrepresentation of Sean Parker is entirely intentional.
Upon the film's release, numerous critics attacked it for its Very Loosely Based On a True Story approach to factual accuracy. Of the particular inaccuracies singled out, the film misrepresenting Sean Parker was a frequent one, and indeed the film's Sean Parker is nothing like the real Sean Parker: he's much more good-looking, more overtly devious and charming, and for purposes of simplifying the narrative is a composite of Sean Parker and Shawn Fanning, the latter of whom traditionally gets the credit for founding Napster (although it has been acknowledged that the filmmakers did get one thing right: the real Sean's extremely rapid speech).

But these criticisms tend to overlook the film's use of Rashomon Style. Of all the main players involved in Facebook (Mark, Eduardo, Sean, the Winklevosses and Divya Narendra), Sean is the only one not heard from directly. Hence, the audience's impression of Sean is entirely dependent on the highly questionable testimony from Mark and Eduardo. Thus, testimony from Mark paints Sean as an extremely charismatic, stylish, competent and visionary individual; testimony from Eduardo paints him as devious, conspiring and obnoxious. That the film's Sean Parker is more attractive and well-dressed than the real Sean could be explained away as simply a method to visually represent his charisma and sense of style.

And this deliberate misrepresentation of Sean is not merely done for its own sake, but for the purpose of affirming one of the film's core themes: the sense of disconnect and dislocation engendered by the Internet and social networking. When Mark and Eduardo are about to meet Sean in the restaurant, both are already familiar with his history: Eduardo points out that he was something of a wild card, having crashed out of both Napster and Plaxo. Mark points out that he was partially responsible for founding the companies. Thus, both Mark and Eduardo both have preconceptions of what Sean is like, because of the Internet. Perhaps the most telling piece of dialogue in this regard is when Eduardo has just arrived in California and is welcomed into the house by Sean:

"Sean: You think you know me, right?

Eduardo: I've read enough."

Eduardo's preconceptions of Sean have been formed based on what he's read on the Internet and thus his testimony pointedly only recalls things that confirm his preconceptions. Thus, just as the audience has no idea what Sean is really like, neither do Mark and Eduardo, because of the way that what they have read on the Internet has shaped how they view the world.
 * In fact, it's been pointed out that, when Mark narrates his creation of Face Mash, all of his explanations about what he was doing were completely technically accurate and plausible (closely based, in fact, on records of the actual event). On the other hand, when he's running students through a hacking contest for the internship positions, his technical explanations don't make any sense. Why? Because that scene is Eduardo's recollection of events, and Eduardo isn't a programmer. Each scene was intentionally designed to reflect the biases and specific ideas of the person who was telling about it.