https://www.theregister.co.uk/2019/08/14/facebook_messenger_conversations/

Facebook secretly employed hundreds of contractors to listen to clips of its addicts' private voice calls to transcribe parts of conversations its AI software couldn't understand.

Basically, if you allowed Facebook Messenger to automatically transcribe your voice calls, and the antisocial network's machine-learning code couldn't work out what you said during your nattering, an audio clip of your voice would be shared with a human to listen to and figure out what was said, according to Bloomberg on Tuesday.