If you advertise on Reddit.. who are you advertising to? If I am in /r/sport (for example), does the advertiser know my gender, my prefered sports (through other subreddit subscriptions), my other interests, my age, my schedule, my social connections/network, my upvoting history, what sports stuff I am most likely to click, whether I like funny stuff, gossipy stuff, serious stuff, factually dense stuff?

Reddit could build profiles on people or even ask for that information in the signup form but their users will not like that one bit.

In addition I would be worried about the age demographic lowering dramatically (my opinion, not a fact) in recent years.