How do you know all those tweet-happy celebrities are the real deal? 50 Cent, Keith Olbermann, Christopher Walken, and Britney Spears are just a few of those with questionable Twitter identities.

50 Cent, né Curtis Jackson III, has hired a Web ghostwriter, Chris Romero, also known as Broadway, to post updates on the message-broadcasting service for him, the New York Times reports.

What we have here is a rapper with a stage name who relies on another guy with an extra moniker to represent his real, authentic self to fans. Is your head spinning yet?

Last week, Keith Olbermann ranted to thousands of MSNBC viewers that Twitter was the "worst person in the world" for allowing an unknown person to "perpetuate a fraud" by impersonating him on the service as "@keitholbermann." (On Twitter, users address each other with the "@" sign.)

The only problem: It turns out that an MSNBC employee had registered the account on Olbermann's behalf. Before the account's owner went private and deleted all of its updates, the @keitholbermann account seemed to be sending updates similar to the official Twitter feed for Olbermann's show, Countdown. Here's Olbermann's rant:

Then there's the curious case of Twitter's Christopher Walken, whose fake account has been disavowed by the actor — and yet is as real as it gets. The fakester's work is reminiscent of Dan Lyons's Fake Steve Jobs in its zany yet realistic insights into the inner life of a famous person. The clever impersonator, as yet anonymous, recently granted an interview to The Wrap. His explanation of his work as @cwalken:

I simply enjoy writing for voices other than my own. When I post a "cwalken" update I am hoping to write something as I would imagine it spoken by Christoper Walken. The politics, tastes and observations are my own. That is — I am not trying to speak for Christopher Walken. I am simply borrowing his voice and reworking my words in his cadence.

Some people crochet, I do this.

For some, pretending to be a celebrity on Twitter is a hobby. But for others, it's a business — like the small army of people Britney Spears employs. Until recently, Joseph Nejman was one of them. He's now dismissive of the practice:

"It's O.K. to tweet for a brand," he said, remarking how common it is for companies to have Twitter accounts, "but not O.K. for a celebrity. But the truth is, they are a brand. What they are to the public is not always what they are behind the curtain. If the manager knows that better than the star, then they should do it."

What Nejman does not mention: Spears's management operation fired him for incompetence in January, after the Harvard grad posted a clumsy help-wanted ad looking for a ghost Twitterer on his alma mater's alumni website. (In a major no-no for celebrity help-seekers, Nejman actually named Spears as the client in the ad, a move which Hollywood veterans scoffed at as likely to attract deranged fans instead of real talent.) Now that he's no longer being paid to pimp out Britney Spears on Twitter, Nejman doesn't think anyone should!

But in posing as a social-media expert instead of a fired hack, Nejman isn't doing anything worse than most people on Twitter, celebrity or not. A few are honest about their fakeness, like Technology Review editor-in-chief Jason Pontin, who wrote last August of his growing Twitter fixation:

But I will never use social technologies quite as the young use them, because I do not thrill to continuous attention and I value my privacy. Thus, the Jason Pontin who occupies the social space is a constructed persona, designed to be unchallengingly personable, humorous, and thoughtful. I am none of those things very often. The preoccupations of that Jason Pontin are professional: he thinks about emerging technologies all the time. And I never broadcast the substance of my inner life, because I know it would become insubstantial the moment I did.

Wall Street Journal editor Julia Angwin likewise recently figured out the point of Twitter: It is not about living your life with friends in real time. It is about promoting your work to gullible strangers.

That's the grand irony of Twitter: Even the real people on the service are fake. They are their own simulacra. No one actually lives their life 140 characters at a time. What we do is turn ourselves into works of fiction. Who's real? Who's not? Who cares?