Zynga Inc filed paperwork on Friday for an initial public offering, the latest in a series of hot social media companies to seek capital in the U.S. public markets.

The company, which is behind a series of popular games on Facebook, said it hoped to raise up to $1 billion. It did not specify the number of shares it was planning to sell or give an expected price range.

A source previously told Reuters that Zynga's IPO could raise $1.5 billion to $2 billion and could value the company at $15 billion to $20 billion.

The amount of money a company says it plans to raise in its first IPO filings is used to calculate registration fees. The final size of the IPO could be different.

Zynga is the company behind popular games on Facebook including FarmVille and Mafia Wars with more than 232 million monthly active users, according to the filing.

It is the top game publisher on Facebook, and while it is free, its revenue comes mainly from selling virtual items such as tractors and weapons that are used to enhance game play.

In the three months ended March 31, Zynga's common stockholders broke even on revenue of $235.4 million. During the same period, Zynga reported adjusted earnings before interest taxes depreciation and amortization of $112.3 million.

Zynga offers an alternative to investors seeking to expand beyond traditional videogame companies, which have seen their share prices erode in recent years. Zynga's games, which don't require hardware and are played mainly on the Facebook platform, have been eating into the $60.4 billion global video game industry, which consists largely of action or sports games played on consoles and TV sets.

Online video game revenue is expected to reach $18 billion by the end of 2011 and $26.9 billion by 2015, according to DFC Intelligence, a firm that does market research on interactive entertainment.

Zynga has expanded rapidly through small acquisitions at the rate of about one a month in the last year, as it seeks to dominate the games market and expand internationally.

Underwriters are being led by Morgan Stanley and Goldman Sachs.

(Reporting by Clare Baldwin and Liana B. Baker; Editing by Lisa Von Ahn and Gunna Dickson)