Craigslist only monetizes housing and job listings, which are probably its two most competitive verticals.

Reddit's monetization issue is ironic, because it's incredibly valuable to marketers: organic/viral growth via Reddit is easy and lucrative. The problem is that Reddit's platform is designed in such a way that there's really no need to go through advertising channels: why pay money to get off on the wrong foot with your audience when you could go through the normal means of the site? (I'm not sure what the admin's stance is on subreddit takeovers, but I'd imagine those would be an avenue worth going down.)