May 17, 2012 10:45am EST – Social networking site Pinterest (PrivCo Private Company Ticker: PINTP), an online scrapbook which allows users to virtually “pin” photos, lists, and ideas to their page, has raised $100 million in a funding round led by Japanese internet giant Rakuten, Inc.

Also participating in Pinterest’s $100 million round were existing investors Andreessen Horowitz, Bessemer Ventures, and FirstMark Capital. Pinterest purposely chose Rakuten rather than new VC firms for its ability to expand Pinterest internationally into Japan and Rakuten’s 17 other global markets.

This hotly contested funding round values Pinterest at a sizeable $1.5 billion. Like Instagram, which Facebook acquired for $1 billion last month, Pinterest has yet to establish a proven monetization model for its site. Pinterest’s last $40 million funding round in October valued the site at $200 million.

Pinterest is currently the 3rd largest social media site in the United States and was the fastest ever to reach 10 million unique monthly visitors. Pinterest recorded 18.7 million unique visitors in March 2012.

Pinterest’s popularity initially caught on with the Midwestern, middle-aged female demographic, however, has quickly moved into mainstream. With over 80% of pins “re-pinned”, Pinterest’s average user spends 89 minutes per month on the site.

In a statement, Rakuten CEO Hiroshi Mikitani said, “We see tremendous synergies between Pinterest’s vision and Rakuten’s model for e-commerce.”

“If a business model is in motion, this could be the first of many big valuations for Pinterest. However, PrivCo still feels Pinterest is an attractive target, and its most likely outcome is an acquisition by company like Facebook or Google,” said PrivCo CEO and Founder Sam Hamadeh in a statement.

PrivCo had previously detailed ‘The Top 5 Reasons Facebook Should Acquire Pinterest - And Quickly’. Shortly thereafter, Facebook acquired comparable mobile company Instagram.