what revenue? where are the customers? how much are they paying to acquire them? how much affiliate revenue do they actually receive per transaction? how do they retain customers? how are they planning to overcome the non-existent barriers to entry in that space?
scorpion032's comment: "Steve has enough from the Reddit sale." is dead on. think about it.. these guys could self-fund the project without breaking a sweat and yet they still sold 20 point equity stake. why? --diversifying risk or pr, really.
either steve realizes hipmunk's a longshot and isn't quite sure that he and his co-founder's street cred will actually play out in terms of long-term customer acquisitions and retention so he's cashing out now while the handful of early adopters (their "e-following") will still look like an aggressive growth curve), or he's trying to get some free press. basically.
edit: i say "longshot" in the sense that i feel it's unlikely that their competitors will do nothing to adapt to any serious hipmunk rise... so unless their competitors are clueless-- which i doubt they are-- and unless they develop some sort of non-replicable competitive advantage-- which i dont see happening-- i think they'll have a hell of a time gaining any ground.