Facebook (Nasdaq: FB), the No. 1 social networking site, said it had elected Chief Operating Officer Sheryl Sandberg to its board of directors.

Sandberg, 42, a former VP of Google (Nasdaq: GOOG), the No. 1 search engine, becomes the sole woman director of the Mountain View, Calif.-based company.

Before the $16 billion initial public offering, the California State Teachers' Retirement System and the National Organization for Women criticized Facebook and CEO Mark Zuckerberg, 28, for not having any women directors.

Sheryl has been my partner in running Facebook and has been central to our growth and success over the years, Zuckerberg said. She joins seven men as directors.

Facebook shares fell 99 cents to $32.06 on Monday. They were priced at $38 in the May 17 initial public offering.