Facebook is the world’s largest social network, with more than 800 million active users around the world, and roughly 200 million in the United States, or two-thirds of the population.

Created in 2004 by Mark Zuckerberg in his dorm room at Harvard, Facebook grew from being a quirky site for college students into a popular platform that is used to sell cars and movies, win over voters in presidential elections and organize protest movements. It offers advertisers a global platform, with the exception of China, where Facebook does not operate.

Facebook took its first step toward becoming a publicly traded company on Feb. 1, 2012, when it filed to sell shares on the stock market. The service is on track to be the largest Internet initial public offering ever — trumping Google’s in 2004 or Netscape’s nearly a decade before that. In its filing, Facebook said it was seeking to raise $5 billion. The company will seek to have the ticker “FB” for its shares, but did not list an exchange.

Many close to the company say that Facebook is aiming for a far greater offering that would value it near $100 billion. At that lofty valuation, Facebook would be much bigger than many longer-established American companies, including Abbott Labs, Caterpillar, Kraft Foods, Goldman Sachs and Ford Motor.

Trading of the stock is expected to begin by late May 2012.

The filing sheds some light on how its meteoric run has turned the upstart into a formidable money-maker. The company, which makes the bulk of its money from advertising and the sale of virtual goods, recorded revenue of $3.7 billion in 2011, a 88 percent increase from the prior year. During that period, Facebook posted a profit of $1 billion. It is still a fraction of the size of rival Google, which recorded revenue of $37.9 billion in 2011, but many analysts believe Facebook’s fortunes will rapidly multiply as advertisers direct more and more capital to the Web’s social hive.

Facebook, unlike any other site, has come to define the social era of the Web. More than a portal, its value lies in its dynamic network of social connections and the massive amount of information shared by its users. Facebook, in many ways, is a data processor, archiving and analyzing every shred of information, from our interests, to our locations, to every article and link that we “like.” The collection of data is a potential goldmine for advertisers, keen to better understand and target consumers.

The social network has become something like an economy onto itself, fostering businesses like the music service Spotify. Game-maker Zynga, which went public late in 2011, generates more than 90 percent of its sales from Facebook.

More Advertising, More Dollars

Facebook’s hundreds of millions of users could soon be faced with a lot more advertising — in their newsfeed, on their mobile devices and even when they log off.

On March 1, 2012, the company announced a new suite of advertising products intended to insert more ads into Facebook’s traditionally clean interface and to take more advantage of mobile ads, where the company has struggled. The announcement was made at the company’s first marketing conference, held at the American Museum of Natural History in Manhattan.

For users, the announcement could mean many more ads on Facebook. For advertisers, the effort offers a chance to reach more users in more places.

Despite aggressively courting Madison Avenue for the last few years, Facebook has been an anomaly in the world of digital advertising. The ad units offered less creative options for advertisers who want to, say, take over the site’s home page or add moving text to an ad. Rather, the value in Facebook’s ads was in their data and personalization.

The potential for more ad dollars was reflected in the company’s first filing for a public offering in February. At the time, analysts said the company was expected to be valued at $75 billion to $100 billion. But according to the filing, Facebook made only $3.7 billion in revenue last year, the bulk of that from advertising.

Until now, advertisers were largely limited to a variety of ad spaces that were positioned on the right side of the Facebook home page, in addition to creating their own Facebook pages. The company said a new set of “premium ads” will run at different points in the site, with a special emphasis on ads running throughout a user’s mobile feed.

Questions About Privacy in the Internet Age

As the world’s largest social network, Facebook faces intense scrutiny from consumers, courts and regulators worldwide over how it handles the data it collects from its 845 million users. But as a company preparing to go public, it is under pressure to find new ways to turn that data into profit.

Facebook’s biggest stumbling block has been privacy. The company has repeatedly alienated users over privacy — as in the case of the 2007 controversy over Beacon, a tool that automatically posted on Facebook what its users did or bought on other sites. It has also faced lawsuits over the use of its members’ “like” endorsements in ads and drawn scrutiny for a facial recognition feature.

Facebook announced a settlement agreement in November 2011 with the Federal Trade Commission, which accused the company of having deceived its customers about privacy settings. After the F.T.C. order, Mark Zuckerberg conceded in a blog post that the company had made “a bunch of mistakes,” but he said it had already fixed several of the issues cited by the commission.

The scrutiny is at its most intense in Europe. Regulators in Ireland, where Facebook has its European headquarters, have already demanded that it give users greater control over their information. A proposed Europe-wide law goes much further by requiring Facebook, along with every other online business, to expunge every bit of personal data at a consumer’s request.

In the United States, Facebook faces government audits for the next 20 years about how it collects and shares data, along with an assortment of lawsuits that accuse the company of tracking users across the Web.

The company’s flubs in this area reveal a fundamental tension in the way sophisticated ad-supported sites work. Consumers’ time and information are effectively the price they pay for free Web services. Facebook allows its users to keep up with far-flung friends and family, for instance, in exchange for that information. Google allows anyone to search for anything, so long as the company can serve up ads based on those searches.

At the moment, the battle is on between those two models of making money from online advertising. If Google’s search engine cast the Internet as an instrument of solitary exploration, Facebook requires its users to share what they do with their Facebook “friends.” In some ways, the Facebook offering is a test of how valuable the social model of the Internet could be.

Some Changes on the Site

To address privacy concerns, Facebook made changes in August 2011 that it said were aimed at helping users get a grip on what they shared.

After the changes were introduced, every time Facebook users added pictures, comments or other content to their profile pages, they could specify who could see it: all of their Facebook friends, a specific group of friends or everyone who has access to the Internet. Icons indicating this replaced the former, more complicated padlock menu. Similar controls were applied to information on users’ profile pages.

In December 2011, Facebook started rolling out a revamped profile design called Timeline that makes a user’s entire history of photos, links and other things shared on the site much more accessible with a single click. That could be when many of Facebook’s 800 million members realized just how many digital breadcrumbs they had been leaving on the site — and on the Web in general.

The old Facebook profile page showed the most recent items a user posted, along with things like photos of them posted by others. But Timeline creates a scrapbook-like montage, assembling photos, links and updates for each month and year since they signed up for Facebook.

For better or worse, the new format is likely to bring back old memories. Going forward, it could also make it harder to shed past identities — something that people growing up with Facebook might struggle with as they transition from high school to college, and from there to the working world.

Analysts said Timeline was a significant evolutionary shift for Facebook. For starters, linking Facebook more closely to memories could make it harder for people to abandon the service for rivals.

A Threat to Google?

Facebook has increasingly been seen as the only company to pose a threat to Google, which has used its dominant position in search and online ad placement to expand into most corners of the Web.

But as a closed network, Facebook with its oceans of content is out of the reach of Google’s search engines, and some analysts think the personal recommendations made through Facebook networks could become a rival to the algorithm-based results pioneered by Google.

In one sign of how much Facebook regards Google as a competitor, The Daily Beast reported that in May 2011 Facebook had hired a public relations firm to persuade reporters and privacy advocates to write stories critical of a new Google service, Social Circles.

With the potential for legal and regulatory clashes growing along with its influence, Facebook has layered its executive, legal, policy and communications ranks with high-powered politicos from both parties, beefing up its firepower for future battles in Washington and beyond.

Facebook’s History: Disputed Origins

The company’s rise has been marked by controversy. 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.

The company has denied the allegations. Another Harvard classmate, Aaron Greenspan, claims that he created the underlying architecture for both companies, but has declined to enter in a legal battle.

“The Social Network,” a movie released in 2010 about Facebook’s tumultuous origins, offered what A. O. Scott of The New York Times 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 such portrayals of the company’s founding are 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.