I can answer this as an advertiser. Most reddit users don't log in or subscribe to subreddits, and thus reddit has a very small targetable userbase. Like, if I

I can answer this as an advertiser. Most reddit users don't log in or subscribe to subreddits, and thus reddit has a very small targetable userbase. Like, if I want to serve an ad to business owners, I can serve it to subscribers to /r/smallbusiness but that audience exhausts very quickly. All of FB's users are logged in and can be targeted by interest.