Social media giant Facebook has finally appointed a woman to its board of directors, in the form of chief operating officer Sheryl Sandberg.

Sandberg, who joined Facebook in 2008, is a vocal critic of the gender imbalance in Silicon Valley's upper echelons. She recently made headlines by announcing that she leaves work promptly at 5:30pm every day in order to spend time with her children.

Sandberg will join Facebook founder and chief executive Mark Zuckerberg on the board, as well as Marc Andreessen, Erskine Bowles, James Breyer, Donald Graham, Reed Hastings and Peter Thiel.

In her current role, she oversees the company's business operations including sales, marketing, business development, legal, human resources, public policy and communications.

“Sheryl has been my partner in running Facebook and has been central to our growth and success over the years,” said Zuckerberg 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.”

Before joining Facebook, Sandberg was vice president of global online sales and operations at Google, where she built and managed the online sales channels for advertising and publishing operations for consumer products worldwide.

She has also served as chief of staff for the United States Treasury Department under President Bill Clinton and began her career as an economist with the World Bank. She received BA and MBA degrees from Harvard University.

Sandberg already serves on the boards of The Walt Disney Company, Women for Women International, the Center for Global Development and V-Day.

“Facebook is working every day to make the world more open and connected,” said Sandberg. “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.”

Facebook has come under criticism in the past for having no female directors. Back in April, women’s rights advocacy group UltraViolet submitted a petition signed by 53,000 people, demanding that Facebook add a woman to its board of directors.

The California State Teachers' Retirement System (CalSTRS) also wrote an open letter to the company in February, complaining about the gender imbalance.

Shaunna Thomas, co-founder of UltraViolet, told the Financial Times that Sandberg's appointment was “a good first step,” but said she hoped Facebook did not think “one woman on the board was enough” to reach gender parity.