But one facet in particular is causing a stir among corporate security lawyers and venture investors: the range of shareholder lockup expirations contained in the Palo Alto, Calif.-based company's IPO prospectus.

The golden rule among lockup agreements, which mandate when investors may sell shares in a newly public company, has been 180 days after the shares priced in an IPO. A couple of quarters of financial reports from the newly public company has long been considered a suitable period to prevent a flood of shares onto the public market from management and large stockholders.

Facebook has flouted this unwritten rule. In its IPO prospectus, which was filed in February, the company said that an undisclosed number of shares (held by unidentified investors) can be sold on the public market after 91 days, 181 days, and 211 days.

The document did provide a little detail: Russian Internet company DST Group Ltd. and its former affiliate, Mail.ru Group Ltd., which has invested more than $500 million in Facebook during the past three years, agreed to be restricted to selling 50% of their holdings after 180 days and 75% after a full year. The firms, founded by Russian billionaire Yuri Milner, will be allowed to sell all of their shares after 18 months. A Milner spokesman did not respond to a request for comment.

Venture capitalists view the six separate lockup expirations in the Facebook prospectus as the opening of a Pandora's box, and they predict the precedent promises to complicate negotiations between investors and startup boards in future IPO candidates.

"I like to keep it simple -- if you don't, you are inviting questions that take you away from the company's core message," said Robert Ackerman, managing director and founder of Palo Alto, Calif.-based Allegis Capital. "If you do something different like this, you better have a damned good reason with a simple explanation."

There have been no lawsuits filed against Facebook over the varying lockup periods. But some lawyers think this looser policy could lead to litigation. "It gives the impression that a subset of investors is able to sell their shares within a shorter lockup period than the typical 180-day lockup for all stockholders," said John Park, a partner with Morgan, Lewis and Bockius LLP in Palo Alto.

"This may create dissatisfaction among the smaller stockholders and potentially increase the risk of derivative securities litigation. There used to be world peace -- 180 days was it; that was the understood norm."

While Facebook's lockup arrangement seems to be intended to ensure an orderly distribution of its shares on the public market, it could also create conflict as a locked-up group of shareholders observe those that are free to sell their shares do so at a high price. What if those investors forced to hold on to their shares find the market value of their holdings much lower when they are allowed to sell?

"It's a matter of fairness," Park argued. "If the early sellers get out before a significant price decrease, it could be a situation where there is an embarrassment of riches."

The notion of staggering lockups has even struck companies that have already gone public. Social networking video game developer Zynga Inc. on Wednesday announced that it would sell up to $400 million in stock via a public offering, well-before the May 28 expiration of its lockup period. The company, whose shares went public in December, said it would release the selling stockholders from the lockup to "facilitate an orderly distribution of shares" and boost the number of Zynga shares outstanding. The company's shares debuted at $10 apiece, and traded at $13.26 recently.

LinkedIn Corp., which marked the first high-profile social media IPO when it debuted in May 2011, saw its shares drop in November when its lockup expired.

Worries over flooding the market with Facebook shares might be the easy explanation for the company's unique lockup arrangements, but it isn't contained in any public documents Facebook has filed to this point. A company representative did not respond to a request to discuss the matter.

Of Facebook's hundreds of private investors, Palo Alto-based venture firm Accel Partners holds the largest stake, with 11.4% of the company's shares. If Facebook's IPO values the company as high as the estimated $100 billion, the $12.6 million invested by the firm in 2006 would be worth a cool $11.4 billion.

DST Global holds 5.4% of the company. Overall, the largest Facebook shareholder is co-founder and CEO Mark Zuckerberg, who owns 28.4% of the company. The company's co-founder and first head of engineering, Dustin Moskovitz, who left Facebook in 2008, holds a 7.4% stake.

Some VCs were willing to concede that layered lockup expirations could be potentially positive -- a way to further avoid a rush of shares onto the market. "There is a fair perspective that having different lockup periods in an IPO could make lockups and liquidity releases more effective and orderly," said Gus Tai, a general partner with Menlo Park, Calif.-based Trinity Ventures.

Yet Tai also voiced concerns about the complications introduced by numerous lockup expirations.

"You need to look at it on a case-by-case basis; whether having different lockups makes it all more confusing and harder to manage is a trade-off that a board and a company need to figure out," he said. "If I'm on a board, I like to say stick with the market norms."

From a broader perspective, the lockup issue might temper the widely held hope that a Facebook IPO could reignite the tepid IPO market.

"Going public is so complicated, expensive and painful already," said Tim Draper, managing director and founder of Draper Fisher Jurvetson, who added that the challenges are pushing many companies to remain private and list on alternative trading platforms such as Xpert Financial, which is backed by a personal investment from Draper.

"This added complexity might be the final straw," he said.