Facebook Chief Executive Mark Zuckerberg cited Desmond-Hellman's experience shaping public policy and operating public companies.
Desmond-Hellman will serve on the board effective immediately, but will have to be elected by shareholders, along with the other Facebook directors, at the company's annual meeting in June.
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Facebook Chief Operating Officer Sheryl Sandberg joined Facebook's board in June 2012, a month after the company's rocky initial public offering.
The world's No. 1 online social network became the only U.S. company to debut with a market value of more than $100 billion. But its shares plunged more than 50 percent in the months after the IPO on concerns about its long-term money making prospects.
Facebook shares have rebounded roughly 56 percent from their 52-week low, finishing Wednesday's regular trading session at $27.45.