From the company that said it never would. Tumblr is planting a little billboard next to your dashboard stream.
Here's Tumblr's CEO, David Karp, talking to the LA Times in 2010:
We're pretty opposed to advertising," Karp said this week in an interview at the company's Park Avenue office. "It really turns our stomachs."
And then here he is again, a week ago, talking to Ad Age:
Our attitude toward all this attention on Tumblr, which we could very easily throw a Google Adsense on and be profitable tomorrow -- that's so far down the list, I mean, we're selling our desks to avoid that, it's a complete last resort.
Aaaaaaand then here he is at the Ad Age Digital Conference yesterday:
I was probably being an idiot then.