Why would you think StackOverflow doesn't care about ad revenue? They both run ads and want to make money by helping recruiters reach logged in users.
Yep but quantity is not an ally to reach this goal, they look for people that have a good track record and are recognized by their peers. They don't care if the student that look for answers on SO is not registered.
Registering non participating members is only a trick to inflate your user base when communicating. What really matter is active users, and they will register don't worry.
You never wondered why there is very few fake account on stackoverflow as compared to ad based social network?