From everything I've read 2012 was a year of monetization firsts for Tumblr - some native ad models for promoted posts, some branded campaigns on topic pages. Even if these were showing promising results it's very hard to get to $100m booked in 2013. I worked on Google AdSense for three years and have seen YouTube monetization scale from 2007 to now. You need not just inventory but headcount and infrastructure. You need brand and agency relationships which move from their experimental budgets to endemic spend.
However, according to him, the $100 million figure could well be just a journalist’s guess.
It seems clear though that 2013 is the year Tumblr will have to earn money. An objective of $100 million seems very high. Let’s see what they come up with.