"You're forever a statistic, worth a few pence to Facebook, and a few quid to everyone else who buys data."
Umm, no. Your data is worth whatever value can actually be extracted from it. That's not the cost.
FB make money from your data by packaging it up and selling it on to someone who thinks that FB have under-valued it. They then either package it up or use it to guide the placement of ads. In the latter case, they are selling ad-placement to a third party who reckon that the second party has under-valued it. And so on, until it reaches someone who doesn't sell it on.
Like a pyramid scheme, the last person in the chain is basically financing everyone else. The question is, does the last person actually get more value out of the product than they paid for it? Maybe. Can they prove that? In the case of advertising, there are so many uncontrolled variables that the answer is certainly "no".