I highly doubt the advertising brings in a quarter of what the donations do. I wonder why people don't check into how much it costs to pay for a site before they rush over to their paypal accounts.
I was surprised to find out how cheap it is. I paid between $200 and $300 for the software. The hosting costs less than $10 a month and that is WAY mire than we need. We use right around 2% of what we pay for.
Let's say for shits and giggles CZ or CF costs $100 per month to host and $300 to renew(both amounts are on the high side) every year. The members more than pay for both of those sites to exist. Someone would have to show me what ads pay, cause even for as much traffic as both those places have, I don't see how the ads would rake in money. Maybe they do. Maybe the ads pay for the site and the members create a lot of profit.