- Facebook is more about real time communication and chat. Reddit is more a forum oriented system with a lot of Subreddit (very hard to monetize). Facebook bought Whatsapp because it was eating up it's market which is the real time communication.
- Facebook have more information about it's users (age, location, friends, ethnicity, interests..etc) which make it more valuable for marketer > ie more money. Reddit knows less about its users and their online ad serving system is in the beginning stage at most. Reddit users also tend to know more about technology (adblocks) and are more sensitive toward ads.
- Reddit need to take a lesson from Digg or even the recent changes to Slashdot. Introducing anything new to this kind of communities might mean their downfall. The market place seems like a good ideas but not fully explored.
Twitter, an arguably much more similar platform to Reddit, is on the other hand the site people sign up to be marketed to
If Reddit could have been bigger by copying the Facebook model, they'd have done it by monetizing more conspicuously, earlier, and disregarding their more vocal users.