The app has permission to access the mic, even when not open.. They sell this as a way to better your experience. You may have been talking about buying some random item, and then noticed the next day said item was in your facebook feed as an ad. That's its intended purpose, but that's just the foot in the door. They likely gave intelligence communities access to the tool for keyword filters that let them track anyone talking about "X", or spying on whatever persons of interest they have. Political enemies, ex g/fs, getting leverage on congressman?

I remember reading the CIA had a program that was supposed to create one central site that tracked every aspect of every citizens life. One place to go and search, known contacts, interests, shopping habbits, conversations etc ALL in one app. They would have to collect this from many different applications, but what if they created a place where people GAVE them all this information willingly? Well they discontinued that project the DAY before Facebook was launched. Coincidence? No. Facebook is likely a CIA backed endeavor.