By A. Ananthalakshmi

(Reuters) - Silicon Valley startup Pinterest raised $100 million from a group led by Japanese online retailer Rakuten Inc, valuing the company at about $1.5 billion and underscoring the huge investor appetite for social-networking companies.

The news comes a day before Facebook starts trading as a public company that could be worth more than $100 billion.

Pinterest, an online scrapbook where users can "pin" images and follow others, has grown from less than one million users in May 2011 to about 20 million in April, according to IT research house comScore.

The three-year old company, led by co-founder Ben Silbermann, had been seeking financing for several weeks, but many investors were wary about the billion-dollar valuation, venture capitalists said.

That valuation puts Pinterest beyond the reach of all but a handful of potential acquirers, probably putting it onto a path for an eventual IPO rather than an acquisition.

Still, the company attracted many interested parties.

"It was one of the most hotly contested and sought-after financing events in Silicon Valley in quite some time," Mike Jaconi, a Rakuten executive who was involved in the negotiations, told Reuters.

Jaconi said Rakuten's presence in 18 countries helped it get a piece of Pinterest as the U.S. company was looking to expand internationally.

Rakuten's CEO Hiroshi Mikitani will take on an advisory role at Pinterest, Jaconi said.

A Pinterest spokeswoman said the Japanese company would integrate the "pin it" button on its websites.

Rakuten, which offers services ranging from online golf reservations to internet banking, was joined in the investment by existing Pinterest shareholders Andreessen Horowitz, Bessemer Venture Partners and FirstMark Capital.

Rakuten is aggressively expanding into global markets. Last year, it bought Canadian ebook company Kobo for $315 million and UK online shopping site Play.com for about 25 million pounds ($39.53 million).

HIGH VALUATIONS

Valuations for consumer-oriented tech startups have been rising dramatically, as illustrated by Facebook's $1 billion purchase of photo-sharing application Instagram last month.

Pinterest's $1.5 billion valuation is up sharply from $200 million late last year.

The company, like Instagram, has no significant revenue.

A Pinterest spokeswoman declined to disclose the company's revenue, saying it is not focused on monetization for now.

($1 = 0.6324 British pounds)

(Reporting by A. Ananthalakshmi and Sarah McBride; additional reporting by Bijoy Koyitty; Editing by Joyjeet Das and Don Sebastian)

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Check out the slideshow to see some of the most popular Pinners:
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  • #11: Ez Pudewa

    Around 907,000 Pinterest users follow <a href="http://pinterest.com/ezpudewa/" target="_hplink">Ez Pudewa's page</a> of 54 boards and more than 3,100 pins.

  • #10: Bright.Bazaar

    <a href="http://pinterest.com/will_uk/" target="_hplink">Bright.Bazaar</a> (a.k.a. Will) certainly has a bright Pinterest page with 24 colorful boards and more than 5,400 pins. He has more than 924,000 followers.

  • #9: Michael Wurm

    With 42 boards and 4,210 pins, <a href="http://pinterest.com/inspiredbycharm/" target="_hplink">Pinterest user Michael Wurm</a> has collected around 940,000 followers.

  • #8: Christine Martinez

    <a href="http://pinterest.com/chrisem/" target="_hplink">Christine Martinez's creative Pinterest page</a> features 43 boards and about 6,000 pins, winning her roughly 979,000 followers.

  • #7: Daniel Bear Hunley

    Around 1,175,000 people follow <a href="http://pinterest.com/danielhunley/" target="_hplink">Daniel Bear Hunley's Pinterest page</a>, which contains 29 boards and about 2,300 pins.

  • #6: anna h

    User <a href="http://pinterest.com/anna_h/" target="_hplink">anna h</a>'s profile has garnered more than 1,200,00 followers with 52 boards and about 3,400 pins.

  • #5: Caitlin Cawley

    Pinterest user <a href="http://pinterest.com/caitlin_cawley/" target="_hplink">Caitlin Cawley</a> has more than 1,235,000 followers, 34 boards and about 2,000 pins on her page.

  • #4: Mike D

    With 27 boards and roughly 4,300 pins, <a href="http://pinterest.com/tempspaz/" target="_hplink">Mike D's Pinterest page</a> has attracted almost 1,270,000 followers.

  • #3: Maia McDonald

    About 1,317,000 users follow <a href="http://pinterest.com/maia_mcdonald/" target="_hplink">Maia McDonald</a> on Pinterest. Her page has 35 boards and more than 11,000 pins.

  • #2: Jennifer Chong

    <a href="http://pinterest.com/jchongdesign/" target="_hplink">Jennifer Chong's</a> Pinterest page has 55 boards, almost 3,000 pins and roughly 1,550,000 followers.

  • #1: Jane Wang

    Pinner <a href="http://pinterest.com/janew/#" target="_hplink"> Jane Wang</a> has nearly 2,844,000 followers on Pinterest. Her prolific pinning has spawned 83 boards and over 15,000 pins.

  • Pinterest Becomes Third Most Popular Social Network

    An Experian report claims Pinterest is now the third most popular social network, behind Facebook and Twitter.