I share your dislike of pretend businesses. (I call it "playing office dress-up".) But Tumblr had $14 million in revenue last year. I suspect they could be in the black on that, and if not, they're within striking range. So I don't think this is a "mud pie" company.
Further, businesses like Twitter, Facebook, and Tumblr have strong incentives to defer seeking revenue as long as possible. The businesses will only succeed at large scale. Time spend on early revenue is a distraction from what really matters, which is pleasing users. Better to wait until you have a large share of the market. So I think it's dangerous to look at Tumblr's current revenue and assume that a) that says much about their future revenue, or b) the company doesn't have a pretty solid revenue plan that they're waiting to roll out.
Further, businesses like Twitter, Facebook, and Tumblr have strong incentives to defer seeking revenue as long as possible. The businesses will only succeed at large scale. Time spend on early revenue is a distraction from what really matters, which is pleasing users. Better to wait until you have a large share of the market. So I think it's dangerous to look at Tumblr's current revenue and assume that a) that says much about their future revenue, or b) the company doesn't have a pretty solid revenue plan that they're waiting to roll out.