Where are you hiding, Zuckerberg, tweeted Salman Rushdie

San Francisco: The writer Salman Rushdie objected when Facebook tried to use his name as it appeared on his passport, and nowhere else.

Would Facebook, he scoffed, have turned J. Edgar Hoover into John Hoover?

"Where are you hiding, Mark?" he demanded of Mark Zuckerberg, Facebook's chief executive, in one post. "Come out here and give me back my name!"

The Twitter verse took up his cause. Within two hours, Mr. Rushdie gleefully declared victory: "Facebook has buckled! I'm Salman Rushdie again. I feel so much better. An identity crisis at my age is no fun."

Mr. Rushdie's predicament points to one of the trickiest notions about life in the digital age: Are you who you say you are online? Whose business is it - and why?

As the Internet becomes the place for all kinds of transactions, from buying shoes to overthrowing despots, an increasingly vital debate is emerging over how people represent and reveal themselves on the Web sites they visit. One side envisions a system in which you use a sort of digital passport, bearing your real name and issued by a company like Facebook, to travel across the Internet. Another side believes in the right to don different hats - and sometimes masks - so you can consume and express what you want, without fear of offline repercussions.

The argument over pseudonyms - known online as the "nym wars" - goes to the heart of how the Internet might be organized in the future. Major Internet companies like Google, Facebook and Twitter have a valuable stake in this debate - and, in some cases, vastly different corporate philosophies on the issue that signal their own ambitions.

Facebook insists on what it calls authentic identity, or real names. And it is becoming a de facto passport vendor of sorts, allowing its users to sign into seven million other sites and applications with their Facebook user names and passwords.

Google's social network, Google+, which opened up to all comers in September, likewise wants the real names its users are known by offline, and it has frozen the accounts of some perceived offenders.

But Google has indicated more recently that it will eventually allow some use of aliases. Vic Gundotra, the Google executive responsible for the social network, said at a conference last month that he wanted to make sure its "atmosphere" remained comfortable even with people using fake names. "It's complicated to get this right," he said.

Twitter, by sharp contrast, follows a laissez-faire approach, allowing the use of pseudonyms by WikiLeaks supporters and a prankster using the name @FakeSarahPalin, among many others. It does consider deceitful impersonation to be grounds for suspension.

The debate over identity has material consequences. Data that is tied to real people is valuable for businesses and government authorities alike. Forrester Research recently estimated that companies spent $2 billion a year for personal data, as Internet users leave what the company calls "an exponentially growing digital footprint."

And then there are the political consequences. Activists across the Arab world and in Britain have learned this year that social media sites can be effective in mobilizing uprisings, but using a real name on those sites can lead authorities right to an activist's door.

"The real risk to the world is if information technology pivots to a completely authentic identity for everyone," said Joichi Ito, head of the Media Lab at the Massachusetts Institute of Technology. "In the U.S., maybe you don't mind. If every kid in Syria, every time they used the Internet, their identity was visible, they would be dead."

Of course, people have always used pseudonyms. Some, like Mark Twain, are better known by their fake names. Some use online pseudonyms to protect themselves, like victims of abuse. Still others use fake names to harass people.

Facebook has consistently argued for real identity on the grounds that it promotes more civil conversations.

"Facebook has always been based on a real-name culture," said Elliot Schrage, vice president of public policy at Facebook. "We fundamentally believe this leads to greater accountability and a safer and more trusted environment for people who use the service."
Real identity is also good for Facebook's business, particularly as it moves into brokering transactions for things like airline tickets on its site.

Company executives are aware of the difficulties of policing a site with 800 million active users. Plenty of people get away with using fanciful names. And enforcing the real-name policy can present real-life complications. Wael Ghonim, the celebrated Egyptian blogger, used a fake name to set up a popular anti-Mubarak Facebook page. That led Facebook to briefly shut its Arabic version in the middle of the Tahrir Square demonstrations, until a woman in the United States agreed to take it over.

Twitter, on the other hand, has vigorously defended the use of pseudonyms, bucking demands most recently from British government officials who pressed for a real-names policy in the aftermath of the civil unrest across Britain.

"Other services may be declaring you have to use your real name because they think they can monetize that better," said Twitter's chief executive, Dick Costolo. "We are more interested in serving our users first."

At the same time, Twitter is vying with Google and Facebook to be something of a passport authority on the Web. Facebook has the widest reach, offering easy access to sites that deliver things like instant messaging and news. Spotify and MOG, two music sites, require new users to log in with their Facebook identities. This allows those sites to show users what their Facebook friends are listening to.

For consumers, this approach can be a mixed blessing. It means not having to keep track of different passwords for different sites. It also means sharing data about what they are doing online with these emerging "identity intermediaries," as Chris Hoofnagle, a law professor at the University of California, Berkeley, calls them.

"It's convenient," Mr. Hoofnagle said. "But do you want Facebook and Google to know where you're going?"

As for Facebook's crackdown on Mr. Rushdie, the company would not explain how it happened but admitted it was a mistake. "We apologize for the inconvenience this caused him," Facebook said in a statement.

Mr. Rushdie, who once lived incognito because of death threats, has more recently been busy revealing himself on Twitter. He had to fight for his online name there as well. An imposter was using the Twitter handle @SalmanRushdie earlier this year, and Mr. Rushdie had to ask the company for help reclaiming it. Now his page bears Twitter's blue "Verified Account" checkmark and quotes Popeye: "I yam what I yam and that's all that I yam."