Declaring that Facebook was built to be not just a company but also to “accomplish a social mission,” the idealistic young creator of the world’s largest online social network launched what will be the biggest Internet IPO in Silicon Valley history Wednesday, as the Menlo Park-based company revealed the long-secret details of its business for the first time.

The projected $5 billion initial public offering was only half the size many analysts had been expecting, but represents the official start of Silicon Valley’s most widely anticipated stock offering in nearly a decade. The IPO filing also marked a continental divide for Facebook, between its birth as a wildly successful startup, and its future as one of the valley’s corporate giants.

Facebook in its IPO filing revealed that it now has 845 million users, nearly half the world’s Internet users. And it reported $1 billion in profit on $3.7 billion in revenue last year, making a company once derided for having no business plan one of the valley’s top profit machines.

Industry sources expect the stock offering will set Facebook’s overall value at $75 billion to $100 billion. The papers filed Wednesday do not specify how many shares will be sold, their offering price, or the date they will be offered; the company is expected to release more details in coming weeks.

The S-1 filing also revealed how dominant founder and CEO Mark Zuckerberg’s control of the company is, with Facebook’s structure of two classes of stock ownership and a highly unusual arrangement that gives Zuckerberg 57 percent of the voting power of Facebook stock.

And Zuckerberg, in a founder’s letter to shareholders that echoed the anti-Wall Street ethos of Google’s (GOOG) founders’ letter in 2004, declared that he plans to exercise that power to do more than maximize Facebook’s profit.

“Facebook was not originally created to be a company. It was built to accomplish a social mission — to make the world more open and connected,” Zuckerberg wrote in the letter, adding, “Simply put: we don’t build services to make money; we make money to build better services.”

Idealist or not, the IPO will certainly make Zuckerberg one of the America’s wealthiest men.

The Facebook founder owns 533,801,850 shares of stock; based on the company’s last determination that those shares were valued at $29.73, his stake in the company would be worth more than $15.8 billion. He also has options to buy 120 million more shares.

Once Facebook goes public, however, his stake is likely to be worth far more. Sam Hamedeh, CEO of the New York financial analysis firm PrivCo, said people close to the deal have told him Facebook plans to sell its shares at up to $40 apiece. That number could rise amid investor demand.

Like several other tech companies, including Google, Facebook currently has a dual-class stock structure that the company intends to keep after the offering. Owners of Class B shares receive 10 votes per share, compared with just 1 vote per share for owners of Class A shares.

Zuckerberg will control a significant portion of both share classes. And he has been laying the groundwork for that extraordinary level of control for years, with one early investor saying Wednesday that the founder long ago began requiring that investors in the company give him the power to vote their shares. While the investor said he had never before encountered such a demand — especially from the 20-something CEO of a fledgling company — he didn’t hesitate to say yes.

“It’s unusual, but I think people are convinced that Mark has been a terrifically effective and inspiring leader,” the investor said, who spoke on condition of anonymity because of federal regulations. “There were no concerns that he was able to do the job. He’s been so right, and he’s been singularly focused on the product. We saw the potential as unlimited.”

Facebook first turned a profit in 2009, when it earned $229 million on $777 million in sales, according to the filing. And the company is not hurting for cash. At the end of 2011, Facebook had $3.9 billion in cash and marketable securities, up from $1.8 billion at the end of 2010.

The bulk of Facebook’s revenue — 85 percent — comes from advertising, but a growing portion of it comes from fees through “Facebook Credits,” largely those related to processing payments for virtual and other goods purchased on the site.

Despite the strong revenue numbers, there were also some signs in the Facebook filing that could be a potential concern for investors.

“I don’t think anyone would say this isn’t an extraordinary business, but it’s also probably going to be valued like one that will do phenomenal things for a long time,” said Joe Magyer, a senior analyst with the Motley Fool. “If they fall anywhere short of sustained awesomeness, the stock is going to disappoint.”

Facebook has seen its user base grow phenomenally over time, but Wednesday’s filing indicates that its user growth has slowed markedly, and Facebook said it would have to concentrate on developing nations outside Europe and North America to expand its membership in the future. That means Facebook will need to find ways to get users to spend more and more time on the social network, increasing revenue by exposing them to more advertising.

And while about 425 million people use Facebook on a mobile device each month, the company highlighted the move to the mobile Web as a substantial risk. Facebook generates virtually no revenue from mobile now, and mobile ads are less lucrative per ad than desktop ads. Meanwhile, rivals Google and Apple (AAPL) control the mobile software platforms most smartphone users rely on to access the social network.

“That’s the No. 1 risk to the Facebook story, in my mind,” said Lou Kerner, an analyst with SecondShares. Internet “use has migrated to mobile and they are not monetizing it.”

Only a handful of companies worldwide have debuted on the public market with total market capitalization of $70 billion or more, according to research firm Dealogic. None of them were U.S. or tech companies.

If expectations for the value of Facebook’s IPO prove true, the company would be worth far more, for example, than Hewlett-Packard’s (HPQ) market value of $55.5 billion. Google, by comparison, has a market capitalization of more than $188 billion, while Apple is worth more than $425 billion.

Facebook is much smaller than those companies, in both revenue and workforce. But it has been on a rapid growth curve.

The number of Facebook employees increased by 50 percent last year, as the company ended the year with 3,200 employees — including those at the company’s new headquarters on the former Sun Microsystems campus in Menlo Park, the company revealed in Wednesday’s filing.

Over the next month, the Securities and Exchange Commission will scrutinize Facebook’s filing, before the company begins a series of presentations to potential investors. Trading of Facebook stock, under the symbol “FB,” might start somewhere toward the middle of the year. Facebook has not yet decided on which stock exchange it will trade.

And while the atmosphere at Facebook’s Menlo Park campus was very much business as usual Wednesday, with no signs of popping champagne corks or spontaneous celebrations of any kind, some large investors were preparing to celebrate.

“It’s a tectonic shift in the history of capitalism,” said Manuel Henriquez, CEO of Palo Alto-based investment fund Hercules Technology Growth Capital, which last month bought 300,000 shares of Facebook on the secondary markets for $10 million. “A college dropout, 27 years old, brings Wall Street to its knees.”

Bay Area News Group staff writer Joshua Melvin contributed to this story. Contact Mike Swift at 408-271-3648. Follow him at Twitter.com/swiftstories, Facebook and view his Google+ profile.