Anonymous users see ads. Authenticated users don't. It costs $5 for an account.
They have run funding drives but, to date, $5/user keeps the lights on and provides a small crew of moderators each a modest stipend.
Edit: spelling, grammar.
I don't think we actually know that as a fact, it's just that not much else has been tried.
When someone starts letting me pay for groceries with my browser history sign me up.
Would be like standing in the town square screaming your thoughts into a loud speaker then being upset that everyone knows what your thinking.
Good idea isn't it?