Facebook Inc founder and CEO Mark Zuckerberg will sell 41.4 million shares worth about $2.3 billion to pay a tax bill, as part of an offering by the social network of 70 million Class A common shares.

Facebook shares, which have doubled in value this year, were down 4 percent at $53.54 before the bell.

Zuckerberg's sale will reduce his voting power to 56.1 percent from 58.8 percent, the company said in a filing on Thursday.

Facebook said it expects Zuckerberg to use most of the net proceeds from the sale to satisfy taxes related to the exercise of an option to buy 60 million Class B shares.

Each Class A share is entitled to one vote, while Class B shares are entitled to 10 votes and are convertible at any time into one Class A share.

Board member Marc Andreessen will also sell 1.65 million Class A shares, while Facebook will sell 27 million.

JP Morgan, BofA Merrill Lynch, Morgan Stanley and Barclays are joint bookrunners for the offering.