Tumblr launched in February 2007, with the tagline “Blogging made easy”. Its first accounts specialised in art, media and porn (14 of the top 20 search keywords containing the word “tumblr” are still associated with adult blogs, according to SEOBook.com). Around 42 per cent of all original posts are photos. But Tumblr is growing up, fast: the site expanded its user base by 900 per cent in the year to June 2011. In 2010, it served under two billion monthly page views; now, it generates about 14 billion, more than Wikipedia or Twitter.

— Tumbling on success: How Tumblr’s David Karp built a £500 million empire (Wired UK)