Hurray! An answer: "With Google Adsense, which Mudcat uses, the site owner is paid per click, or per thousand times the ad is displayed - I don't know which."

It seems to me the latter must be "which". I presume that people who do not like to see ads will not click on them if forced to see them. Thus, if the Mudcat were paid by the click, it would lose nothing if such people blocked ads. But then, why not set it up that way? If what the advertiser wants is clicks, why shouldn't that be what the advertiser pays for? I will not repeat my guess.