SAN JOSE, Calif. — In a milestone for one of Silicon Valley’s hottest companies, Facebook on… SAN JOSE, Calif. — In a milestone for one of Silicon Valley’s hottest companies, Facebook on Wednesday filed papers announcing a $5 billion initial public offering of stock in the world’s biggest social networking business.

The stated size of the offering is smaller than expected, after weeks of speculation by analysts and industry observers who predicted Facebook might seek up to $10 billion. But documents indicated the figure is preliminary; the company could revise the numbers as it prepares to begin selling stock this spring.

With the filing of its initial prospectus, which states that it intends to trade under the symbol “FB,” Facebook is officially launching Silicon Valley’s most widely anticipated stock offering in recent years. For the Menlo Park, Calif., company and its 27-year-old co-founder Mark Zuckerberg, it represents an official transition from wildly successful startup to one of the Valley’s corporate giants.

Industry sources expect the stock offering will set Facebook’s overall value between $75 billion and $100 billion. The papers filed Wednesday do not specify a price or how many shares are being offered, although they indicate the company will have two classes of shares. The company is expected to release those numbers in coming weeks.

But the filing does provide an initial glimpse into details of Facebook’s operations and finances, which the privately held company has closely guarded until now. For example, the company disclosed that it earned $1 billion in profit on $3.7 billion in revenue last year, after sales rose 88 percent from 2010.

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.

Zuckerberg, who serves as CEO, owns 28.2 percent of Facebook — a share that is significantly higher than previous reports have indicated.

Given the company’s high profile and profitability, however, his compensation was relatively modest last year. It totaled $1.5 million, which includes the $692,679 it cost Facebook for Zuckerberg and his friends and family to fly on a chartered airplane. Facebook paid Zuckerberg $483,333 in salary last year and gave him a $220,500 bonus.

But the compensation of Zuckerberg’s fellow executives was far less modest. Facebook paid its chief operating officer, Sheryl Sandberg, $30.9 million last year, an amount that includes stock awards worth some $30.5 million. Chief Financial Officer David Ebersman received $18.7 million last year, which includes $18.3 million worth of stock awards. And engineering chief Mike Schroepfer received $24.7 million last year, including $24.4 million in restricted stock.

Facebook has also seen its user base grow phenomenally over time, although the filing indicates that the growth has slowed markedly.

At the end of 2011, Facebook had 845 million users who accessed the site at least once a month and 483 million users who visited the site on a daily basis. That was up from 608 million monthly users and 327 daily visitors at the end of 2010. Facebook had 360 million monthly users and 185 million daily visitors at the end of 2009.

Zuckerberg highlighted that growth in a letter to potential investors, in which he vowed, “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.”

He went on: “We hope to improve how people connect to businesses and the economy. We think a more open and connected world will help create a stronger economy with more authentic businesses that build better products and services.”

While there have been larger IPOs in the tech industry, an initial offering of $5 billion would still outrank any other Internet-based company’s debut. Google’s initial offering in 2004, at $1.7 billion, held the previous record for the largest Internet IPO by a U.S. company.

Morgan Stanley was listed as the lead underwriter of the stock offering, after beating out Goldman Sachs for the lucrative fees that go with that position. Goldman and several other firms will also participate, however.

By some estimates, Facebook’s IPO is expected to create between 500 and 1,000 millionaires among Facebook employees who have been granted company stock — as much as one-third of its workforce.

And although some contrarians suggest the company is overvalued, the IPO is expected to cement Facebook’s standing as one of Silicon Valley’s most valuable companies — worth far more, for example, than Hewlett-Packard’s market value of $55.5 billion. Google, by comparison, has a market capitalization over $188 billion, whereas Apple is worth more than $425 billion.

Facebook is much smaller than those companies, with just about 3,000 employees and an annual revenue estimated at $4 billion. But it has been on a rapid growth curve.

The Facebook website was initially created for Ivy League college students when Zuckerberg launched it from his Harvard dorm room in 2004. A few months later, Zuckerberg moved to Palo Alto, Calif., and opened the rapidly growing social network to the general public two years later. Facebook now has more than 800 million users around the world and is expected to pass 1 billion later this year.

With that growth, Facebook has steadily increased its share of the rapidly expanding market for online advertising — emerging as a powerful rival to older, established Internet companies like Yahoo and Google.

Advertisers in the United States alone spent $36 billion to reach online audiences in 2011, up from $26 billion in 2010, according to the research firm eMarketer. The firm estimates Facebook’s share of that revenue grew from 4.6 percent in 2010 to 6.4 percent in 2011.

Google still dominates the U.S. ad market, with an estimated share of nearly 41 percent. But analysts say Facebook poses a greater threat than those numbers suggest, since its users represent a captive audience for advertisers — an audience that spends vast amounts of time chatting, sharing pictures and playing games within Facebook’s “walled garden” of sites, outside the reach of Google’s search engine.

With its “Like” button and a host of other ways to determine users’ hobbies, interests and preferences, Facebook offers the ability to reach a highly targeted audience, Internet analyst Martin Pyykkonen of Wedge Partners wrote in a report this week. At the same time, he noted, a majority of Facebook users are in the 18- to 49-year-old age group, which is coveted by the same big advertisers that use television to reach mass audiences.

Facebook also has a growing source of revenue from its “Facebook Credits” virtual currency, which people use to buy services that run on Facebook’s network, such as items for Zynga’s popular social games like “FarmVille” and “CityVille.”

Peter Delevett of the San Jose Mercury News contributed to this report.