Maybe for a small subset of the total advertising spend, but for the big players, they can't always be sure which part of their revenue can be attributed to Facebook and which part can be attributed to other sources or which part can be attributed to their sheer scale (everyone knows about Coca Cola, but that won't stop the company from spending big on advertising). Companies like Procter and Gamble started scaling back their Facebook ad spend; they had no idea what their FB ROI was until they actually started scaling back and they realized that it was essentially $0.