Another way to look at it is: If X is a really good website in its category, and Google consistently puts X above other websites in search results for that cate

Another way to look at it is: If X is a really good website in its category, and Google consistently puts X above other websites in search results for that category, the user experience on Google is better with X than without X. This will lead to more users using Google more, and contribute to all the effects that make Google money. So even if Rap Genius is not bringing revenue to Google, Google certainly can derive value from Rap Genius that contributes to revenue. However, since the effects are indirect, it's not easy (though not impossible) to put a monetary value on this relationship.