As long as the CDN is configured to transmit its files with a far-future expires header (which Google and Microsoft's CDNs are), the CDN owner can't accurately

As long as the CDN is configured to transmit its files with a far-future expires header (which Google and Microsoft's CDNs are), the CDN owner can't accurately track usage at all. In a perfect world, the CDN would only handle one HTTP request per user per expires period (+1 year on Google and Microsoft's jQuery CDN). In reality, the file will be pushed out of or cleared from caches for various reasons, but they still persist long enough to make tracking end-usage of the CDN meaningless.