No, the advertisers in radio and tv are in the same boat as the stations themselves... no one really knows how many people really saw/heard an ad. They just both choose to trust a third party (neilson/arbitron) to give them the ratings. It just isn't possible to know for sure how many people saw an ad from a broadcast. Cable/sat and DVRs make it a bit easier, but only a bit.

On the web though, it's ridiculously easy to verify each request that there is no need for the trusted third party.