24/7 Wall St.

Shares of Facebook Inc. (NASDAQ: FB) traded down in the premarket Thursday morning, following the announcement that the company will have a follow-on stock offering of 70 million shares. CEO Mark Zuckerberg is selling 41.35 million Class A shares in the offering primarily “to satisfy taxes he will incur in connection with his exercise, in full, of an outstanding stock option to purchase 60 million shares of Class B common stock.”

Each share of Facebook’s Class B stock is entitled to 10 votes and is convertible at any time into one share of Class A common stock. Following the offering announced this morning, Zuckerberg will control about 62.8% of Facebook’s outstanding capital stock.

The filing notes that Zuckerberg plans to make a gift of 18 million Class B shares to an unnamed recipient this month. The shares will be converted to Class A shares when they are donated.

The company is selling just over 27 million shares in the offering, and selling shareholders are selling nearly 43 million, including Zuckerberg’s 41.35 million shares. No other selling shareholders are named in the company’s filing. Facebook will receive no proceeds from the sale by selling shareholders and plans to use its proceeds from the rest for working capital and general corporate purposes.

While it is well past time that Facebook’s lockup period expired, it is probably worth reminding investors of what can happen to a stock’s price when insiders are first allowed to sell stock following an initial public offering. Shares of Gogo Inc. (NASDAQ: GOGO), an Internet and wireless connectivity provider for in-flight services, fell more than 15% when its lockup period expired on Wednesday, and one of its private equity sponsors dumped all of its 27.6 million shares.

Twitter Inc. (NYSE: TWTR) has two lockup dates in front of it. Some employees may sell 9.9 million shares beginning February 15, while company executives, directors and large shareholders have to wait until May 6, six full months following the company’s IPO. Six months was the typical lockup period for a long time, with occasional exceptions like Facebook, which allowed some shareholders to sell after a lockup period of 90 days.

Facebook’s addition to the S&P 500 becomes effective at the close of trading tomorrow, so having another 70 million shares outstanding is probably not a very big deal for a company with more than 1.8 billion shares already out there.

Less than an hour before the opening bell on Thursday, Facebook stock was trading down about 2.7% at $54.07 in a 52-week range of $22.57 to $55.89.