[wayne]

Facebook's North America revenue is around $50 a quarter per user ("ARPU"), which is ~$15/month. You'd have to charge quite a bit to clear that, given most users won't pay. (Plus 30% to pay for App Store fees.)

Reddit probably did the math and figured the churn was worth it to get everyone seeing ads. Facebook is much more optimized for ads, but optimizing Reddit's ads is probably easier than getting more people to pay. The latter would probably require paywalling more which is just as icky.

[paxys]

They are planning to charge third party apps ~$2.5/mo per user (aggregate amount based on average usage), so that is already quite a bit less.