Facebook, the world's largest social network, announced in July 2010, that it had 500 million users around the world. The company has grown at a meteoric pace, doubling in size since 2009 and pushing international competitors aside. Its policies, more than those of any other company, are helping to define standards for privacy in the Internet age.

The company, founded in 2004 by a Harvard sophomore, Mark Zuckerberg, began life catering first to Harvard students and then to all high school and college students. It has since evolved into a broadly popular online destination used by both teenagers and adults of all ages. In country after country, Facebook is cementing itself as the leader and often displacing other social networks, much as it outflanked MySpace in the United States.

A movie about the tumultuous origins of Facebook, "The Social Network," offers up what A.O. Scott called "a creation story for the digital age and something of a morality tale, one driven by desire, marked by triumph, tainted by betrayal and inspired by the new gospel: the geek shall inherit the earth."

Facebook has strenuously, and Mr. Zuckerberg more quietly, asserted that the portrayal of the company's founding is fiction. And Mr. Zuckerberg disputed the characterization of him in the film, though in a New Yorker magazine profile, he acknowledged having indulged in a bit of sophomoric arrogance.

Facebook's rise has been marked by strings of controversies. Three other Harvard students maintain that they came up with the original idea and that Mr. Zuckerberg, whom they had hired to write code for the site, stole the idea to create Facebook. Facebook has denied the allegations. A long-running lawsuit is pending. Another Harvard classmate, Aaron Greenspan, claims that he created the underlying architecture for both companies, but has declined to enter the legal battle.

But it has also come to be seen as one of the new titans of the Internet, challenging even Google with a vision of a web tied together through personal relationships and recommendations, rather than by search algorithms. In a major expansion, Facebook has been trying to spread itself across other websites by offering members the chance to "Like'' something — share it with their network — without leaving the web page they're on.

Like other social networks, the site allows its users to create a profile page and forge online links with friends and acquaintances. It has distinguished itself from rivals, partly by imposing a spartan design ethos and limiting how users can change the appearance of their profile pages. That has cut down on visual clutter and threats like spam, which plague rivals.

In May 2007, Facebook unveiled an initiative called Facebook Platform, inviting third-party software makers to create programs for the service and to make money on advertising alongside them. The announcement stimulated the creation of hundreds of new features or "social applications" on Facebook, from games to new music and photo sharing tools, which had the effect of further turbo-charging activity on the site.

Facebook is increasingly finding itself at the center of a tense discussion over privacy and how personal data is used by the Web sites that collect it. Facebook users, privacy advocates and government officials in many countries have lobbed vociferous complaints that some new features or settings were privacy violations. 

Facebook has pushed its users to share more information about themselves. But Facebook users have pushed back, increasingly lobbing vociferous complaints that some new features or settings are privacy violations. The back and forth between Facebook and its users over privacy is gaining importance as the company's growth continues unabated. Facebook's policies, more than those of any other company, are helping to define standards for privacy in the Internet age.

Bowing to pressure over privacy concerns, the company in May 2010 unveiled a set of controls that he said would help people understand what they were sharing online, and with whom.

Facebook's biggest mistake, Mr. Zuckerman said, was failing to notice that as Facebook added new features and its privacy controls grew increasingly complicated, those controls became effectively unusable for many people.