[mingpan]

The other potential concern is that stuff of this sort could get accidentally learned. Suppose there were some sensitive fact about some subset of the population, and suppose it happened to correlate with response to some advertisement. If FB were trying to optimize for better ad targeting and response, they could happen to learn this trait, perhaps without even a human operator realizing it.

Again, these are all relatively unlikely hypotheticals, but it is admittedly unnerving that the potential could exist.