I suppose you don’t see things of this scale often but I would argue that big things like this are happening all the time. I was watching recently some interviews from 2005 about the justification for the US invading Iraq in 2003, and the disparity between what the public was told about WMDs and what the security community believed was maybe on a similar scale (depending on how you view privacy and war). And then the people who made decisions that led to the 2008 financial collapse were doing things that hurt people on what you may consider a similar scale, depending on how you view robbing people of their wealth versus robbing them of their privacy.
Facebook invading our privacy by recording persons of interest or the people en masse would be horrible and in many ways unprecedented, but it wouldn’t be beyond the levels of abuse we have seen from powerful people in the past. I can easily imagine that the app supports hot mic capabilities and that they do turn it on sometimes at least at the request of law enforcement. And then the question is... when else would they turn it on? And would that program ever grow? Would they ever fork the program so each team involved thinks there working on a small project? This is all speculation but I can imagine a situation where it starts small and then grows until it seems like an insane program but everyone involved is accustomed to it.
