Sheryl Sandberg, Facebook’s chief operating officer, will become the social network company’s first woman on its board of directors. (Laurent Gillieron/AP)

On Monday, Sheryl Sandberg was named to Facebook’s board. One of the most common responses: What took them so long? As investor Paul Kedrosky put it on Twitter: “Like most people, I'm sure, my reaction to news that COO Sheryl Sandberg is joining Facebook board is .... Wtf, she wasn't on the board?”

In fact, it’s actually not that common for major companies to have chief operating officers as directors. The number of insiders on corporate boards has dropped substantially in recent years as companies are careful to stock their boards with independent directors. According to the 2011 Board Index from executive search firm Spencer Stuart, the percentage of non-independent directors has fallen from 23 percent to 16 percent over the past decade among the companies in the Standard & Poor’s 500-stock index. (In Silicon Valley, insiders make up about 20 percent of directors.) With the average board of directors comprised of only about 11 members (or a mere 8 in Silicon Valley), that means the average board barely has two insiders: The CEO and then typically an executive chairman or, less commonly, a chief financial officer or COO.

Take a quick look at some of Facebook’s peers. Among the five companies in the Internet services and retailing industry in the Fortune 500 list, the only current executives on the board are the CEO, co-founders or chairmen who are former CEOs. At other Silicon Valley companies, Netflix has tapped its former chief marketing officer, Leslie Kilgore, for its board, and HP includes former EVP Ann Livermore, but it is still pretty rare to see current executive insiders on a company’s board beyond the CEO.

All of which proves how essential Sandberg has been to the company’s success. She is not merely a second-in-command, but, as CEO Mark Zuckerberg said in a statement announcing the news, Sandberg has been “my partner in running Facebook” who “has been central to our growth and success over the years.” She’s been instrumental in shaping the company’s business model and the key figure in building its advertising business.

That’s not all. She’s brought in seasoned pros from her days at Harvard Business School, Google and the U.S. Treasury Department, where she was the chief of staff for none other than Larry Summers. She’s helped to tame what a new book calls Facebook’s “unrepentantly boyish company culture.” All that and she goes home at 5:30 every day. Next stop: Running the world?

In all seriousness, Sandberg’s nomination follows Facebook coming under pressure for having no women on its board. Until it named Sandberg, it was in a severe minority, at least among Corporate America at large: According to Spencer Stuart’s study, 91 percent of S&P 500 companies have at least one woman director. In Silicon Valley, meanwhile, the number is embarrassingly lower: Just 57 percent of these companies have at least one female director.

Investors were starting to take notice. In February, the director of corporate governance for the California State Teacher's Retirement System wrote a letter to Zuckerberg saying, “We are disappointed that the Facebook board will not have any women members. This is particularly glaring in view of the fact that Facebook is going public at a time when there is clear evidence that companies with diverse boards perform far better than the companies with more homogeneous boards.” Sandberg was even mentioned by name.

I usually think adding more insiders to a board is a bad idea, particularly when the board has just eight members. And Facebook’s directors will certainly have their share of issues to deal with as they navigate the company’s rocky debut as a public company. But Sandberg’s integral role in managing the company puts her in a unique position to deserve the nod. Now if only the rest of Silicon Valley will follow suit, and find women—either from inside or outside their ranks—to help lead their companies.  

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