The fact they're raising investment is a pretty clear statement that they want to make money. Just because they're not choosing to focus on revenue at this stage of product development doesn't really mean they have no interest in making money.

Any company needs to decide where their focus should be, Tumblr may well have done the analysis and decided that rather than take their focus away from their core product and focus on monetization (which may only generate a few hundred million in revenue at this stage) it's better for them to focus on their core product now in order to have a chance of monetizing at the billion dollar range in the future.

It's a perfectly rational economic decision.