Traffic growth is a cost, not a revenue. Usually there's some revenue associated with that traffic growth, but there's no indication Reddit's infamously anti-commercial user base is very easy to monetize.
I have no idea how open they are to exploiting it, but I'd bet just having a high-traffic social hub has significant monetary value in the data and analytics. Twitter makes millions selling analytics and feeds, and will probably increase that in the future. I'd personally pay something for all sorts of slices of Reddit data; for example, a real-time feed of pageview info, or a firehose of all comments, would be great. Of course, the real question is whether people with more money than me would pay.