Zynga, maker of FarmVille and other online games, filed paperwork Friday for an initial public offering aiming to raise about $1 billion from the markets. \"By offering our shares to the public we hope to enable Zynga to invest more in play than any company in history,\" Zynga founder and chief executive Mark Pincus said in the company\'s IPO registration statement, posted on the website of the US Securities and Exchange Commission. The number of shares to be sold and the price range for the offering have yet to be determined, Zynga said in a statement. But media estimates said the move could bring the company a market valuation of up to $20 billion. Founded in 2007, the San Francisco-based software company makes notoriously addictive games which run on top of social networking site Facebook, including Zynga Poker and Mafia Wars. The company boasts more than 250 million players per month. Zynga may be hoping to duplicate the success of recent tech IPOs such as that of LinkedIn, the professional networking site whose shares more than doubled in price after they debuted on the New York Stock Exchange in May. Some critics say the recent enthusiasm for tech IPOs is not justified by the companies\' fundamental valuations and may be part of a new tech bubble along the lines of the dot-com boom of 1999-2000.