But Tumblr distinguishes itself as more of an aspirational social network, a place you go to hang out with the people you wish were your friends.
Unlike Facebook and Twitter, the site's homepage is not a drab login but an artistic posture: it features a range of high-resolution artwork; one day this month its top images were an illustration of "Spring Snow" by the late children's author Roger Duvoisin and a series of bleak landscapes by Danish photographer Per Bak Jensen; its tagline is "Follow the World's Creators."
And yes, by joining, you become one of the "world's creators" too. The site now hosts about 53 million blogs, which include teenage diary-type opining, video clips from aspiring auteurs, and many, many pictures of cats. It's an entire universe of people trying to build their personal "brands"- and it's all free.
But every kid must hit adolescence, and every successful start-up must eventually try to monetise. This week it was Tumblr's turn.
With its costs growing rapidly, it has launched its first major revenue scheme, allowing some users to pay for placement on the main page, which could be seen by Tumblr's 110 million monthly visitors.
Many sites have tried to ease users into a new world order where not quite everything is free.
Users will either embrace the change or they will whine and, possibly, flee. But Tumblr's move will not just test the loyalty of its fans or the talents of its founder, 25-year-old internet tycoon David Karp.
If successful, it will provide a way for brands to connect with consumers that is more nuanced than your average internet ad. It would also delight investors, who have poured more than US$125 million ($155 million) into the Manhattan-based company, and provide more oomph to the New York tech community, which is growing up right alongside Tumblr.
Karp admits his company has been burning through cash to keep its growth on track. Choosing when to turn on the profit engines is never clear, but Tumblr had to do something. "We all feel the same pressure," Karp says one sunny afternoon about the feeling at the company. "The numbers are starting to get unforgiving."
Once, around six years ago, Karp's mentor, Fred Seibert, gave him some tough love. Karp had begun working for the television and film producer after dropping out of New York's prestigious Bronx High School of Science at the age of 15.
He had taught himself how to code, and his parents homeschooled him while he worked for Seibert and built Davidville, which he called "an invention company".
It was 2007 when Karp, then 19, turned his interest to blogs. Blogging was already widespread, but existing services, like WordPress, presumed a familiarity with things like HTML that many would-be bloggers didn't have. Karp wondered why no one had invented something simpler, so he did it himself. Bloggers flocked to the site, which he called Tumblr.
The company has come a long way from two employees in a tiny office. It now occupies two floors in a loft building in Manhattan.
Last year Tumblr took in an additional US$85 million in funding and the site has hired around 90 people in the last year (up from fewer than 20 in 2011). To accommodate the more than 16 billion monthly page views - most visitors are between 18 and 35, a slight majority of them women - the site has had to increase its server capacity, its support staff, and its office size.
Karp says there are no immediate plans for an IPO. Until just three weeks ago, Karp had taken a strident position against internet ads, which he said "turn our stomach". Two weeks before his announcement, he told a reporter he considered advertising "a last resort".
Then, on April 18, Karp changed his tune. In a speech at an advertising conference, he spelled out his plans to debut a revenue model for the site. The plan essentially allows users to pay to have their blog posts appear not only on the pages of friends and followers but to the entire Tumblr community. For an array of brands that use Tumblr as a marketing tool, including IBM and Alexander McQueen, this provides an opportunity to push out their message without the stigma of formal ads.
The point is to make the paid content as unobtrusive as possible. Tumblr has spent the last year studying how companies use the site, and has hand-selected the first ones to make use of the new service. (Everyone at the company is at pains not to call it "advertising".) "We worked so freakin' hard to reduce our interface," Karp said in an interview the day after his announcement, "that to just let someone inject their nonsense bit of 'Hey, look over here!' was not something we were going to do," he said. "We want this to be the iPod, not the kitchen sink."
