Facebook was always doing this. It just wasn't proven they were.

VoIP has been around for 25 years. Compressed audio is a tiny amount of packets. It doesn't need HD audio, it just needs enough to recognize words. Enterprise VoIP only needed a 128kb connection 25 years ago to work. On a smartphone, 128kb is nothing and will blend in with background connections. Facebook could record and monitor your audio 24/7 and it won't even be a blip in the network traffic, especially if it does a quick process of the audio before sending it to Facebook.