MENLO PARK, Calif. (AP) - Facebook says it plans to offer 70 million shares of stock for sale that includes more than 41 million shares from chairman and CEO Mark Zuckerberg. The secondary offering of stock comes as the social media network prepares to join the Standard & Poor's 500 index. The Menlo Park, Calif., company said Thursday that the shares will be offered mainly to index funds whose portfolios are based on stocks included in the index. The S&P 500 will add Facebook on Friday after markets close. The index is a list of companies that is meant to be a snapshot of the U.S. economy. Facebook Inc.'s stock went public in 2012. After a rocky start, the company's shares have gained momentum. Its shares fell 4 percent in premarket trading Thursday.