You're right that everything is CPA by proxy. I work for an ad network, and despite selling on a CPM basis, we get evaluated on a CPC or CPA basis frequently. F

You're right that everything is CPA by proxy. I work for an ad network, and despite selling on a CPM basis, we get evaluated on a CPC or CPA basis frequently. For performance campaigns, everything gets broken down to an eCPA for evaluation anyway. Major buyers usually do a small test campaign, look at the performance, then decide if they're going to do a larger campaign or reevaluate. The other example people keep bringing up is branding campaigns, which are a major source of spend online. I agree that for these there may be no measurable action, but that's not to say they're fine with any impressions. The major point for branding campaigns is that site lists, safety, and context matter so much more. If you're trying to do branding, you don't want your ad showing up next to pictures of keg stands or negative articles about your company. Brand campaigns want to be 100% sure that all content will be brand safe, so they only go to big sources they can trust. This is why most major branding campaigns have stringent restrictions, usually excluding all user generated content and long tail publishers. The big branding campaigns are paying $10-20 CPM, but they're buying it on NewYorkTimes.com and ESPN, not on your blog or social media invention.