This is the right move at the right time. $FB just hit a new record high, for one. Also; let's take a look at the overall share structure: 2.45 billion issued and outstanding shares, making this 70 million figure -- of which 42,995,239 are being offered by stockholders -- a drop in the bucket:
"Facebook (FB) is commencing an offering of 70,000,000 shares of its Class A common stock.
A total of 27,004,761 shares are being offered by Facebook, and a total of 42,995,239 shares are being offered by certain selling stockholders, including 41,350,000 shares offered by Mark Zuckerberg.
Standard & Poor's announced that effective as of the close of trading on December 20, it plans to include Facebook's Class A common stock in the S&P 500 Index. Shares of Facebook's Class A common stock will be offered primarily to index funds, whose portfolios are primarily based on stocks included in the S&P 500 Index.
Facebook intends to use the net proceeds of the offering for working capital and other general corporate purposes. Facebook will not receive any proceeds from the sale of shares by the selling stockholders. Facebook expects that the majority of the net proceeds Zuckerberg will receive upon the sale of shares in the offering will be used to satisfy taxes that he will incur in connection with his exercise, in full, of an outstanding stock option to purchase 60,000,000 shares of Class B common stock."