The trackers they build into sites would still exist to track non-paying customers, and they'd have to track the paying customers enough to differentiate from non-paying.
And if you're saying that all of Facebook becomes a subscription, you still end up with the problem that Facebook has pointed out: small sites that earn income from Facebook by implementing their tracking widget would lose that source of income.
I think we should be more vocal about the extensions and products that help block FB (and other) web trackers, to force their model to change, but I think we're a long way away from anything truly effective.