Japanese mobile social gaming firm Gree will aim for 500 million to a billion users worldwide, rivaling Facebook, the company's founder and chief executive said on Wednesday, June 29.

According to Reuters, Gree this year paid $104 million to buy fast-growing U.S.-based mobile social gaming platform OpenFeint, which has 90 million users worldwide and is adding 2.8 more per second.

"We are aiming for 500 million to a billion users, though we don't have a timeframe, or particular countries," Yoshikazu Tanaka said. “Facebook is said to have 700 million, and we are also aiming for that level," he added.

But some analysts say Gree may face a difficult transition to the smartphone era, with Apple restricting integration of social and gaming applications on the iPhone and the spread of free applications on phones based on Google's Android operating system.

Google this week announced its own social networking challenge to Facebook.

Some in the industry expressed surprise at the high price Gree paid for OpenFeint, which made a net loss of $6.6 million on sales of $282,000 in fiscal 2010, according to a Gree statement.

Tanaka said he was confident he could turn the business profitable, though he did not specify the timing, saying it would depend on how much Gree chose to invest.

Gree itself made 19.6 billion yen ($242 million) in operating profit in the financial year to June last year on sales of 35.2 billion yen, and expects to hike operating profit to 27-30 billion yen in the financial year ending on Thursday.

Tanaka also said he expects more growth in the Japanese market, where Gree boasted more than 25 million users as of March 2011.