Edit: since you're asking, proof could be multiple testimonials from ex-engineers working on the project or a peer-reviewed article in a serious journal. Until that this is in the same bin with healing crystals and chemtrails.
Whistleblowers never prosper, at least in the US. Too many examples, take Stingray for a less controversial one. What was the result? Crickets. Exactly what would be the incentive for those engineers?
Myself, I'm a Bayesian and I grew up in a communist country, which means I start from "of course everyone is spying on me, all the time" and require strong evidence to change my mind.
You're expecting 100% of the engineers at Facebook (or previously at Facebook) that know about this to comply with the NDA. And history shows that 100% of Facebook employees will not keep quiet about Facebook doing bad/illegal things.
Where?