That was fast: Six months after he started seeing Alexandra Lenas, Sean Parker is engaged to the singer/songwriter. Parker has made a number of sudden life changes since Justin Timberlake played him in The Social Network, actually.

People magazine got confirmation from Parker's camp today that the former Facebook president is engaged to Lenas. What the celebrity weekly left out is the not-so-happy backstory.

Around the time of the release of The Social Network, or just before, amid the pre-release hype, Parker dumped his longtime girlfriend Kate Jurkiewicz, who he'd been seeing for three and a half years. As of May, Parker and Jurkiewicz were still together. A source close to the couple says he dumped her in the fall, although Parker's spokesman told us the split happened in the spring or over the summer. At the beginning of October, the Social Network was released.

After a brief apparent dalliance with lifecaster turned syndicated columnist Julia Allison—at least one witness spotted the enamored pair kissing at the Manhattan restaurant The Lion in October—Parker then took up with his soon-to-be-fiancée that same month.

In late October, Parker and Lenas were photographed at Heidi Klum's Halloween party. (As you can see, Parker dressed up as Timberlake for the party while Lenas wore a Britney Spears get-up.)

In December, Parker had a birthday celebration at The Lion—which is located just down the block from the townhouse he was renting for most of 2010 and purchased in February for $20 million—and Lanas took to the microphone before assembled guests, publicly snogged with her new honey a few times, and later uploaded 27 pictures of the event to her Facebook account under the title "Happy Birthday to Sean ♥." (See photos below.)

In a flattering Vanity Fair profile last October, Parker insisted he was nothing like the womanizing, backstabbing, drugging guy Timberlake played in The Social Network. "That character really isn't me," he recalled telling Timberlake.

But Parker's playboy reputation preceded the movie and still clings to the tech investor. Some people, including a couple who whisper in our ears, are convinced Parker was dabbling in cocaine last fall; Vanity Fair didn't help matters by writing that he "routinely stays up very, very late, talking intensely about subjects he cares about and/or partying."

Even assuming there's nothing to that, jilting a longtime squeeze for a whirlwind engagement to a hot young thing is only going to further the rakish image of Parker that Timberlake inserted into the national consciousness. Something tells us that, protestations to the contrary notwithstanding, Parker probably doesn't mind too much.

[Photos of Parker and Lenas at the 2011 Oscars and Heidi Klum's 2010 Halloween party via Getty Images]