I just asked a question about fraud, which is something that's really tricky when you're paying a stranger $$$ based on how much they claim they are distributing content. Reasonable answer about the parallels with clickfraud, and how non-zero levels of fraud are ok, but the long-term answer is just aligning people so that the distance between the person distributing the content, and a person paying to receive it, doesn't have room for fraud. It's still a hard problem though!