At the launch ceremony of the Facebook IPO, CEO Mark Zuckerberg tells employees and supporters to stay focused as the company enters a new phase as a publicly traded company.Facebook Inc., operator of the world's largest social networking site, went public Friday on the Nasdaq Stock Market, fetching $42 (USD) at the opening of trade, up from the firm's $38 offering price.Speaking from the location of the company's headquarters in Menlo Park, California, and the site of the Nasdaq opening bell ceremony, company CEO Mark Zuckerberg told a crowd of employees and supporters not to let the occasion of the IPO launch distract from the company's focus on innovation."This all seems like a big deal. Going public is an important milestone in our history but here's the thing: Our mission isn't to be a public company. Our mission is to make the world more open and connected," said Zuckerberg.He thanked the crowd for helping Facebook build "The largest community in the history of the world."It is the world's biggest-ever IPO for an information technology firm, according to Thomson Reuters Corp. The largest IPO in US history was Visa Inc., which raised $17.9 billion in 2008."This is an awesome moment. Remember: stay focused and keep shipping," said Zuckerberg.As investors have strong expectations for Facebook, being used by over 900 million people around the world, the IPO price was set at the upper limit of the expected per-share pricing range.Because Facebook users spend longer time daily on the website compared with the time they spend on other websites, it is said Facebook is well-suited for advertisers to grasp users' interests to offer targeted ads.On the other hand, the company has a profitability issue as more and more people are using the social networking service on smartphones, which have smaller screens and are difficult to display ads.