Facebook doesn't sell user data for money. It sells targeted advertising for money. There is a huge difference.
Because you get more money for a cow than milk over a long time period and can use the cash to expand the cow-selling business?
(And that original metaphor was a bit mismatched, actually.)
FB provides a platform for ads, sure, but it can be used for way more than just ads.
3P tracking can infer who (even specifically) is viewing ads and it knows the social graph by other means. It can also do effective mass psychometric tests. A/B testing infrastructure can be used for more than just optimizing ads ... all of the psychometric dimensions tested by Kogan’s 2013 app can be expressed as embedded imagery and messaging in ads, and the same types of tests can operate at the same scale by integrating 3P data and tracking (some of which certainly originates from policy-compliant FB apps) which already knows the social graph beyond what FB will allow a single app or ad to draw.