Longer sessions. You also have more potential for brand advertising compared to Google Search (which is mostly conversion ads). That being said, it's risky and can ruin your product if done wrong, but I think there are ways to do it right.
What I'm saying is that I believe their ARPU could be higher than Google's, while what I think you're saying is that their cost will also be higher. I agree with that, but where we differ is that I think that while the margin will be lower, there is still potential to make a ton of money there.