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Facebook opened on stock market Friday at $42 a share, the third-largest IPO in history. The social network hit the market a half-hour later than expected after NASDAQ reported "delays" due to massive amounts of data received, and Facebook's stock public two hours after Facebook founder and CEO Mark Zuckerberg rang the Wall Street opening bell Friday. Immediately after opening, Facebook shares dipped below $40 within a half-hour of being on the market, and shares on the tech site Zynga were halted. CNBC looked at the private market data for Facebook, finding that when Facebook first sold shares in April 2008 on SecondMarket, an online financial marketplace where private trades occur, the company was valued at $8.1 billion that year but within a year, the valuation had dropped to $2.6 billion. Zuckerberg addressed Facebook users "you have built the largest community in the world" and said "so let's do this" as he rang the midday bell.