alt Hacker NewsWikipedia is not even in the game you are describing here. Wikipedia does not need billions of users clicking on ads to convince investors in yet another seed. They are an encyclopedia, and if fewer people will visit, they will still be an encyclopedia. Their costs are probably very strongly correlated with their number of visitors.
SO was supposed to be much the same, though. I guess you really do have to get directly funded by users for the model to work.