The per-user Tumblr stat, he said, is greater than the time people spend on competing networks. "It's about a minute and a half higher than a Facebook visit, a few minutes higher than an average Twitter visit," Karp said during an interview at a PaidContent event in New York.
Karp even acknowledged that he's made sure Facebook CEO Mark Zuckerberg is aware of this fact.
"The reason for that is, I don't think that Tumblr is so much better; I think that it's a very different behavior," he said. "It's media -- something to do before checking your email."
Tumblr passed 100 million blogs a few weeks ago. The company now counts 45 billion total blog posts, with 90 million posts created each day. Plus, the network, which now values time spent as a more important metric than pageviews, is responsible for "tens of billions" of minutes per month, he said.
Tumblr, not profitable today, will definitely be at some point, Karp said, though he was quick to add that getting to profitability was not a milestone or high priority for his team. "I don't think any of us are sweating that right now," he said.
Sweat it, they may not, but Twitter and Facebook clearly have a leg up on Karp's 6-year-old service when it comes to making money.