Last update: May 17 at 4:38 a.m. PT.

Pinterest has garnered investment interest from one of the world's largest Internet service providers, in a move that could value the social photo-sharing service at $1.5 billion.

Rakuten, which runs the largest e-commerce site in Japan, announced Thursday that it is leading a $100 million round of investment in Pinterest. Participating in the round are existing investors Andreessen Horowitz, Bessemer Venture Partners, and FirstMark Capital, along with an unspecified number of angel investors.

Rakuten's portion of the funding is in excess of $50 million, according to All Things D, which first reported the investment news (and at the time thought the round might be as big as $120 million). The new round now values Pinterest at $1.5 billion, ATD says.

Beyond the funding alone, Pinterest and Rakuten have entered into a strategic partnership to facilitate Pinterest's expansion in Japan and other markets.

"Our goal is to help people discover things they love, by connecting people through their shared interests. Bringing Rakuten on board gives us an amazing opportunity to move a step closer to this goal," Ben Silbermann, co-founder and CEO of Pinterest, said in a statement.

Launched in March 2010, Pinterest had earlier raised $37 million from investors including Andreessen Horowitz, Bessemer, and FirstMark, which gave it a valuation of $200 million. The site has experienced phenomenal growth in the past few months, doubling its number of daily unique visitors to 4 million since January, according to data released last month by social media marketer Tamba.

In the wake of Facebook's acquisition of Instagram, those numbers make Pinterest ripe for buyers, as my colleague Rafe Needleman points out. "There's really only one startup that emerges as most Instagram-like: Pinterest," Needleman writes. "All the others are battling for position as distant followers."

Founded in 1997, Rakuten had revenues of $4.7 billion in 2011.

Updated May 17 at 4:38 a.m. PT with the official announcement of the Rakuten-led investment round.