In case of facebook where their ad network controls the whole process (SSP + Exchange + DSP) they would only lose, besides the infrastructure cost, the opportunity to make money by serving another ad.

But for the majority of ad serving world it's not that simple. If you're for example a DSP and buy ad impressions from exchanges via real time bidding, you have to pay for the impression even if you can't charge the advertisers for it. So you lose money twice. You don't serve another ad which would make you money and you have to pay for an ad that won't be paid for by the advertiser.