The question for reddit isn't whether or not people enjoy it and want to spend time on it, but whether or not the owners can make money selling those people's attention. The traffic to reddit - while admirably large - is relatively unattractive to most advertisers. "Reach" (impressions/eyeballs) are only important insofar as you're talking to someone who might buy what you're selling (see "relevancy"). The sub-reddit…
"The sub-reddit system could theoretically segment the audience in interesting ways, but other than r/gaming, there aren't many natural industry fits amongst popular sub-reddits." In my opinion this is only true for a few (admittedly most popular) subreddits like r/funny and r/wtf and everything nsfw . Nearly every subreddit that I read has an industry that fits, r/photography -> photo gear, r/parenting -> baby stuff…
The NSFW ones are tricky. On one hand there's a lot of money in porn. But most of the content there is either stuff from porn companies (that would be paying for ads), or amateur stuff, and would an amateur audience like porn company ads?