People track conversions - click through URLs contain tracking query params which track the source. If you convert (download a resource, request a demo, start a trial, buy a product) it gets reported back to Facebook.

There's definitely accountability. The problem is that you can't always blame FB or Google because maybe your targeting is off, your ad copy sucks, you dumped the user off the front page of your website and they bailed, etc. There's a lot more to performance and accountability.