Social network and scrapbook website Pinterest said it took in $50 million from Japan's Rakuten (Tokyo: 4755) empire, which valued the four-year-old company at $1.5 billion.

Pinterest, in Palo Alto, Calif., is one of the fastest-growing websites and previously received funds from investors in Facebook (Nasdaq: FB), the No. 1 social network, whose $104 billion initial public offering is scheduled to be conducted Thursday.

The site said it raised an additional $100 million atop an earlier $40 million investment. Rakuten accounted for half, with follow-on financing from Andreesen Horowitz, a major Facebook investor, as well as Bessemer Venture Partners, First Mark Capital and some undisclosed angel investors.

Rakuten, owner of U.S. sites like and others worldwide, said it plans a close alliance with Pinterest. That deal somewhat resembles how Japan's SoftBank (Tokyo: 9984) helped to fund Yahoo (Nasdaq: YHOO), the No. 3 search engine at inception.

Indeed, Yahoo Japan's market capitalization in Japan's ecommerce sector is only slightly more than Rakuten's.

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We see tremendous synergies between Pinterest's vision and Rakuten's model for e-commerce, said Rakuten CEO Hiroshi Mikitani, 41, whom Forbes ranks No. 6 among the richest Japanese.

Pinterest CEO Ben Silbermann, 29, said the tie gives us an amazing opportunity to move a step closer to a goal of helping people discover the things they love.