Online real estate marketplace Zillow Inc. filed an initial public offering with the Securities and Exchange Commission Monday, hoping to raise about $51.8 million. The Seattle-based company narrowed its yearly loss to $6.8 million after reporting a loss of $12.9 million in 2009 and a loss of $21.2 million in 2008. It reported revenue of $30.5 million in 2010, up 74% from the previous year. The 6-year-old firm said it plans to use proceeds from the IPO for general corporate purposes, including financial flexibility and increased market exposure. Under risk factors, Zillow noted that it has “incurred significant operating losses in the past and we may not be able to generate sufficient revenue to be profitable over the long term,” according to a regulatory filing. As of Dec. 31, 2010, Zillow had an accumulated deficit of $78.7 million and although its revenue grew from $10.8 million to about $30.5 million last year, that growth rate is expected to decline, the company said. Zillow’s financial model depends on advertising revenue that is  generated “almost entirely through sales to real estate agents and brokerages, mortgage lenders and advertisers in categories relevant to real estate,” the firm said in its filing. “While real estate agents participating in our subscription-based Premier Agent program generally commit to contract terms of six or 12 months, we do not have long-term contracts with most of our other advertisers,” it said. The company had about 19 million unique users to its website in March. A month earlier, it began a partnership with Yahoo! Real Estate that provides the largest online real estate network. The company also recently acquired Postlets Inc., a real estate agent and rental property management service. Technology Crossover Ventures and PAR Investment Partners are making a $5.5 million private investment in Zillow, according to the SEC filing. Write to Christine Ricciardi. Follow her on Twitter @HWnewbieCR.