Facebook Inc. investors such as Accel Partners and Goldman Sachs Group Inc. increased the number of shares they’re selling in the social network’s initial public offering, boosting the sale to as much as $16 billion.

Existing holders will offer 241.2 million shares, bringing the total on offer to 421.2 million, a regulatory filing today shows. Accel, the biggest seller in the IPO, raised the amount it’s selling by 28 percent, while Goldman Sachs more than doubled its shares on offer.

Facebook, girding for the largest-ever IPO of a technology company, Tuesday boosted the offering’s price range to as much as $38 a share, indicating a market value as high as $104 billion. While the expanded IPO may mean the appetite for shares is strong, insiders’ decision to pare holdings further may heighten some investors’ concern over Facebook’s earnings growth, said Greenwood Capital’s Walter Todd.

“If the demand wasn’t there, they wouldn’t have upsized the deal,” said Todd, who oversees about $940 million as chief investment officer at the firm based in Greenwood, South Carolina. “On the other hand, when you see insiders unloading their stakes, you start to wonder why. I could see it turning some institutional investors off.”

Accel, based in Palo Alto, California, is now offering 49 million shares, while Goldman Sachs plans to sell 28.7 million. Digital Sky Technologies increased its amount 74 percent to 45.7 million, while Tiger Global Management is selling 23.4 million shares, almost 7 times the amount first offered.

Facebook’s executives and directors increased the amount of shares they’re selling 62 percent to 189.4 million.

Co-founder Mark Zuckerberg, the top shareholder, didn’t boost the amount of shares he’s selling.

Holdings, Lock-Up

Accel will hold about 23 percent of Facebook’s Class A shares following the offering, compared with about 5.9 percent for Goldman Sachs, according to today’s filing. Digital Sky will own about 5.4 percent of the Class B shares after the sale, and Tiger Global will hold about 2 percent.

“Everybody is cashing out,” said Trung-Tin Nguyen, a hedge-fund manager at TTN AG in Zurich. “It’s normal for private equities and venture capitals to cash out, but the obvious question is whether they are stretching it too far.”

Facebook amended the lock-up agreement with Digital Sky and Mail.ru Group Ltd. to allow those investors to sell more of their stock sooner after the IPO. They’re allowed to sell 75.1 million shares 91 days after the IPO and the remainder 366 days after, today’s filing shows.

Milner’s Ownership

Facebook had previously disclosed that the firms could sell 43.2 million shares after the 91-day period. Russian billionaire Yuri Milner, founder of Digital Sky, holds stakes in Mail.ru and Facebook through that investment firm.

Investors are betting that Chief Executive Officer Zuckerberg can overcome slowing sales growth by expanding into areas such as mobile advertising and e-commerce, said Samuel Schwerin, managing partner at New York-based Millennium Technology Value Partners.
“An increasing number of institutional investors are looking beyond the value of the business today and looking at the future growth,”

Schwerin, whose firm oversees $1 billion, including Facebook stock, said yesterday. “Those drivers are extraordinary in size, including international and mobile and commerce.”

Debut Comparisons

Menlo Park, California-based Facebook and its existing holders had earlier planned to offer 337.4 million shares. Before the company raised the price range to $34 to $38 a share, it had offered the stock at $28 to $35. Its underwriters will have the option to buy an additional 63.2 million shares from the company and its holders after the IPO, the company said in the filing today.

At $16 billion, Facebook’s debut would surpass that of General Motors Co. to be the second-largest in U.S. history, excluding so-called over-allotments, which let underwriters buy more shares at a later date, data compiled by Bloomberg show.

GM raised $15.8 billion in November 2010, before expanding the sale to $18.1 billion when underwriters exercised the over- allotment option. Visa Inc. raised $17.9 billion in its 2008 IPO, the biggest in the U.S., and later expanded the sale to $19.7 billion.

Separately, GM, the world’s biggest automaker by vehicles sold, said yesterday that it no longer plans to advertise on Facebook. The social network said last week that growth in advertising sales isn’t keeping pace with gains in users.

The new shares indicate that Facebook is offering about 15 percent of its stock in the IPO, according to the filing. Facebook’s shares are set to price tomorrow and begin trading under the symbol FB on the Nasdaq Stock Market the following day.

Happy 28th

Zuckerberg celebrated his 28th birthday this week, during the final leg of a marketing tour aimed at building demand for the IPO and convincing investors that Facebook can make money from mobile users.

Some institutional investors had balked at buying into Facebook early in the road show over concern about the site’s growth prospects, people with knowledge of the matter said last week. In a Bloomberg Global Poll of more than 1,250 investors, analysts and traders taken before the price range increase, 79 percent said Facebook didn’t deserve such a high valuation.