MENLO PARK, Calif. — Facebook and Chairman and CEO Mark Zuckerberg are selling a combined 70 million shares of Class A stock as the social media company prepares to join the Standard & Poor’s 500 index.

The offering includes more than 41 million shares from Zuckerberg, who also will buy Class B shares that carry more voting weight. After a premarket stock dip of more than 4 percent on the news, Facebook’s stock recovered somewhat and was down less than 1 percent in afternoon trading on Thursday.

The Menlo Park, Calif., company said Thursday that the Class A 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.

At Wednesday’s closing price of $55.57 per share, that would put the total value of the offering, not counting expenses, at about $3.89 billion. Zuckerberg’s offering of 41.3 million shares would generate about $2.3 billion based on Wednesday’s close, not counting expenses.

The company said Zuckerberg will use most of the proceeds from his sale of Class A shares to pay taxes he will incur in connection with exercising an option to buy 60 million shares of Class B stock. He’s also using part of it for charitable contributions.

It’s Zuckerberg’s decision to sell shares that likely led to Facebook’s stock price decline.

That said, Standard & Poor’s equity analyst Scott Kessler noted that the planned sale would only “minimally” reduce Zuckerberg’s stake and voting power.

“We are not concerned by this news,” Kessler said in a note to investors, reiterating a “Buy” rating on Facebook’s stock.