With investors hungry for Facebook shares ahead of a hotly anticipated offering, the social network unveiled a 25 percent increase Wednesday in the number of shares to be sold at the market debut.

The increase comes from current shareholders, who had previously obtained stock in private offerings, seeking to cash in on the hefty price amid a Facebook frenzy.

"This is definitely in response to very strong global investor demand," said Lou Kerner, founder of The Social Internet Fund.

"US institutional demand has been good; the retail and global demand has been overwhelming."

The move by the Internet giant comes one day after it filed paperwork with the US Securities and Exchange Commission raising its initial public offering share price from a range of $28 to $35, to between $34 to $38 per share.

A definitive price was expected on Thursday.

Facebook will reportedly go public hacker style with an all-night software bending bash to culminate at its new California campus with co-founder Mark Zuckerberg remotely ringing the Nasdaq opening bell.

Employees were signing up for a "hackathon" to start late Thursday at Facebook's offices in the Silicon Valley city of Menlo Park and continued until the social network's initial public offering of stock on Friday, according to tech news websites TechCrunch and All Things Digital.

"Hackathons are a big tradition at Facebook," the company explained on a Facebook page devoted to the events.

"They serve as the foundation for some great (and not so great) ideas," the message at facebook.com/hackathon continued.

Kerner said some Facebook insiders may be seeking to take profits now, because they might otherwise have to wait six months under SEC regulations.

Facebook is assured of becoming the most valuable US Web company at the time of an IPO, topping Google's $23 billion valuation in 2004.

Trading is expected to begin on Friday under the symbol "FB" on the technology-heavy Nasdaq.

There was no immediate explanation for the increase in shares, which will come from the current holders of shares in prior private offerings. The latest document says 421 million shares will be issued, up from 337 million. And that could be raised to 484 million if demand is high.

Under the new share plan, Zuckerberg will hold 55.8 percent of the voting power, down slightly from an estimated 57.3 percent. The 28-year-old controls the firm through a dual class stock structure and certain shares that give him a "proxy" for voting.

The net proceeds to the company will remain unchanged at $6.4 billion, the filing with regulators said.

At the midpoint price, the IPO could net some $16 billion for all the sellers at the latest estimate, and at the maximum price more than $18 billion.

Depending on the final value, the IPO would be one of the largest of a US firm on Wall Street, behind the 2008 offering of Visa ($17.8 billion) and possibly above that of General Motors in 2010 ($15.7 billion).

The IPO share price gives Facebook a value of $93-104 billion.

Based on the estimated market value, Facebook would be in the neighborhood of Amazon ($100 billion) and Cisco ($90 billion), but also ahead of Hewlett-Packard ($45 billion) and struggling Yahoo! ($19 billion).

Some analysts predicted Facebook's stock price will jump quickly as the social network finds ways to leverage its membership of 900 million.

The California giant separately agreed Tuesday to a tie-up with Lightbox, an Android photo app, by hiring its developer team. This follows a $1 billion deal to acquire another photo sharing app, Instagram.

The IPO comes amid a wave of tech offerings with mixed success. Groupon, the online deals firm, has lost 34 percent since its November IPO, while the professional social network LinkedIn has soared 150 percent in a year.

In a negative for Facebook, General Motors said it plans to stop advertising at the network because it determined that paid ads had little impact.

"We are reassessing our Facebook advertising, but we remain committed to very aggressive social media strategies and will continue to push content on Facebook through our product and brand Facebook pages," GM spokesman Patrick Morrissey told AFP.

The US auto giant is the third largest advertiser in the United States, with expenditures of $1.8 billion in 2011, according to Kantar Media.

But the consultancy eMarketer says Facebook's share of overall US display ad market revenues grew to 14 percent in 2011 and will likely hit 16.8 percent this year, overtaking Google.