You're right, that fraud was possible, but less so than with TV or Radio or Print. Hell, I spend $$ on radio advertising, even that I have zero way to validate how many people are hearing those radio broadcasts. Advertisers will pay for unvalidated advertising.
If you had something that accidentally blew up on line, then you could have picked up a phone, called a big company that did something related to what this site was about, and ask them to advertise. That's how business works. You can't expect to sustain something that popular with spending zero effort on bringing in some kind of revenue. By using an ad network, that's what you're doing, and that's completely unsustainable. You're producing a service that a LOT of people are consuming, but you're spending zero effort to bring in any income from your work.