Facebook's No. 2 executive, Sheryl Sandberg, has become the first woman to be appointed to the social-networking company's board of directors.

For years one of the most vocal critics of the gender imbalance in Silicon Valley's executive ranks, chief operating officer Sandberg, 42, joined Facebook in 2008 and played a central role in guiding the social networking company to its $16 billion US IPO in May.

Her promotion Monday comes as Facebook seeks to cultivate a more mature image as opposed to the college dorm-room startup reputation that has dogged the company since Harvard dropout Mark Zuckerberg founded it in 2004.

"Sheryl has been my partner in running Facebook and has been central to our growth and success over the years," Zuckerberg, 28, said in a statement. "Her understanding of our mission and long-term opportunity and her experience both at Facebook and on public company boards makes her a natural fit for our board."

Still, even after Facebook elevated Sandberg, the composition of its board remains a continuing point of scrutiny for a young company that has touched countless industries and boasts close to a billion users from every corner of the world.

The California State Teachers' Retirement System, the second-largest largest pension fund in the United States, which owns 36,922 shares of Facebook, applauded Sandberg's promotion but urged the company to "continue diversifying the board toward greater independence and representation of the company's user base."

Apart from Sandberg, the company's board is made up of seven Caucasian men, largely Silicon Valley insiders aligned closely with Zuckerberg. They include Zuckerberg himself; venture capitalists James W. Breyer, Marc Andreessen and Peter Thiel; Washington Post Co. chairman Donald E. Graham; Netflix CEO Reed Hastings; and Erskine Bowles, a former White House chief of staff and the University of North Carolina president emeritus.

"We are optimistic Facebook is on its way to further expanding the board while simultaneously creating the diversity and independence we think is important to the future sustainability of this vibrant company," said Jack Ehnes, CEO of the California State Teachers' Retirement System.

But, he added, "We're not there yet."

Success at Google

Prior to joining Facebook to lead its business operations, Sandberg worked at Google, where she was credited with building the search advertising division into a massively lucrative cornerstone of the web giant's business.

She has been tasked with stoking similar growth at Facebook, which claimed $3.7 billion in revenue last year but is under pressure to justify its $70 billion valuation in the public markets.

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ESPN president George Bodenheimer, far left, Facebook COO Sheryl Sandberg, Walt Disney CEO Robert Iger and Facebook CEO Mark Zuckerberg talk at a media and technology conference at a resort in Sun Valley, Idaho, on July 7, 2010. Sandberg will join seven men on Facebook's board. (Mario Anzuoni/Reuters)

At the same time, Facebook hopes the promotion of one of corporate America's most high-profile women will go some way to allay concerns over its own gender issues, especially given Sandberg's advocacy on the subject.

In February, before Facebook went public, the teachers' retirement system openly urged the company to diversify its board to include women and said then the makeup of the all-male panel was "disappointing."

In April, the women's rights group UltraViolet held a protest outside Facebook's New York offices over the same issue.

The Menlo Park-based company faced embarrassment as recently as last week when the Wall Street Journal published advance excerpts of a memoir by Katherine Losse, an early employee, who recounted being harassed and propositioned by male co-workers until Sandberg intervened when she came onboard.

Sandberg, a married mother of two, has herself criticized women's poor representation on corporate boards and in positions of power generally.

"Women are not making it to the top of any profession anywhere in the world," she told an audience at a TEDWomen conference in December 2010. "A hundred and ninety heads of state; nine are women. Of all the people in parliament in the world, 13 per cent are women. In the corporate sector, women at the top — C-level jobs, board seats — tops out at 15, 16 per cent. The numbers have not moved since 2002 and are going in the wrong direction."

According to Catalyst, a nonprofit group that tracks women's advancement in business, only 16.1 per cent of board seats at Fortune 500 companies in the U.S. were held by women in 2011. In Canada, a mere 14.5 per cent of board seats in private and 10.3 per cent in public companies on the FP500, the list of Canada's 500 largest private and public corporations, were held by women.

Almost 40 per cent of private companies and more than 46 per cent of public companies on the FP500 had no women at all on their boards.

Facebook's public face

In recent years, Sandberg's clout within Facebook has been unquestioned while she has also served as its public face, often in place of the sometimes socially awkward Zuckerberg, who has focused on improving Facebook's product.

Meanwhile, Sandberg has represented the company at events like the World Economic Forum in Davos, Switzerland, where she led a panel on women's career advancement in January.

A former chief of staff to Treasury Secretary Lawrence Summers during the Clinton administration, Sandberg holds seats on the board of Walt Disney Co. and several non-profit organizations.

"Facebook is working every day to make the world more open and connected," she said in a statement. "It's a mission that I'm deeply passionate about, and I feel fortunate to be part of a company that is having such a profound impact in the world."

Under Sandberg's stewardship, Facebook navigated its rocky IPO in May but still faces a litany of growing pains, chief among them are its regular brushes with privacy controversies that threaten to erode the eight year-old service's popularity.

Email switch irks users

On Monday, the social network was again subject to fresh accusations that it had tampered with user privacy by changing the email contact listed for every user to a facebook.com address without notification.

Facebook defended the move, saying it was part of a broader, earlier effort to enhance "consistency" to the service.

"As we announced back in April, we've been updating addresses on Facebook to make them consistent across our site," a Facebook spokeswoman said. "In addition to everyone receiving an address, we're also rolling out a new setting that gives people the choice to decide which addresses they want to show on their timelines."

Facebook shares closed down three per cent at $32.06 on Monday, still more than 15 per cent off of its May offering price despite a rebound in recent weeks.

With files from CBCNews.ca