Pinterest does not disclose user numbers but WSJ reports from comScore:

Pinterest said new investors have joined the round but declined to name them. Past investors, which previously put in $764 million, include venture-capital firms Bessemer Venture Partners and Andreessen Horowitz, hedge fund Valiant Capital Partners and Japanese e-commerce firm Rakuten Inc.

According to The Wall Street Journal’s Billion Dollar Startup Club, only five private, venture-backed companies in the world are valued higher than Pinterest, and one, Indian online retailer Flipkart, is valued at the same amount.Launched in 2010, Pinterest has taken off among the arts-and-craft and design-savvy crowds, gaining particular traction among women. Users are able to “pin” images to pages or “boards” on a range of topics whether if it is to remodel a kitchen or plan a trip.

Pinterest released its first advertising offering—called the promoted pin—in January and has said it is focusing on developing its ad business this year. In recent months, it has released new ad-targeting capabilities and expanded testing advertisements elsewhere on the service.

Pinterest’s loyal and female-centric user base makes it an attractive destination for advertisers, said Matt Britton, CEO of digital marketing agency MRY.

“They have a lot of things going for them,” Mr. Britton said. “Pinterest’s users are the CFO and CMO of the household, moms,” a valuable audience for brand-name consumer product companies. “They can deliver ads to these moms in a way that Facebook and Twitter can’t.”

The company says it aims to use the funds for international expansion. Last year, international users increased more than 135% and now comprise more than 40% of all users, compared with 28% in 2013, Pinterest said.

Pinterest’s unique U.S. visitors reached an all-time high of 75.8 million in January, up 37% from the same time a year earlier, according to comScore. Pinterest doesn’t disclose user numbers.

Worldwide Pinterest monthly uniques estimated at 250 million:

http://www.ebizmba.com/articles/social-networking-websites

Pinterest was the fastest firm to 10 million monthly uniques, which it hit in Feb 2012:

http://techcrunch.com/2012/02/07/pinterest-monthly-uniques/

Pinterest's $11 billion valuation would put the company among the ranks of the most highly valued private tech companies in the world, including Xiaomi (valued at $46 billion), Uber (valued at $41.2 billion), and Palantir and Snapchat (valued at $15 billion).

Read more: http://businessinsider.com/pinterest-raises-367-million-at-11-billion-2015-3