Pinterest was introduced on the web in closed beta status in March 2010.
On October 3 this answer was posted on Quora by Cristian Galletti with 2 votes by Chris McCoy and Elias Bizannes.
"Work on Pinterest began in December 2009 and the site achieved closed beta status in March 2010. The site proceeded to operate in invitation-only open beta. Later, the site made registration possible after an email request. On 16 August 2011, Time magazine published Pinterest in its 50 Best Websites of 2011 column."
Mashable introduced Pinterest to its readers on October 7, 2011 saying that Pinterest revealed “Friday” that it had raised a $27 million round of funding from Andreessen Horowitz.
At the time of the Mashable article, Pinterest had not released the size of their user base but they had 30,000 reviews for the app on the iTune store - a figure that Tumblr took 4 years to get to.
The Pinterest founder is Ben Silbermann
In the comments on that article, Mashable member, Alexandra Gibson said she has been on Pinterest for “years”.
I have a board on Pinterest called "How to Use Pinterest" that you may find useful.
Does anyone have a “How to Use Tumblr” board on Pinterest? I’ll have to search for that.
UPDATE: CNN published an article Feb 6, by Pete Cashmore where he explains why tech sites got caught by surprise by the success of Pinterest.
"The web-based ‘pinboard,’ which launched almost two years ago, barely got a mention on Silicon Valley news sites until six months ago, when early adopters suddenly realized that a site with millions of monthly users had sprung up almost unnoticed by the tech press.
That’s because Pinterest didn’t take the usual route of Web-based startups: romancing early adopters and technology journalists before attempting to “cross the chasm” to mainstream adoption. Instead, Pinterest grew a devoted base of users — most of them female — who enjoy ‘pinning’ items they find around the Web.”