Facebook claims that the only conversations that were transcribed were from those users who selected that option in the Messenger app. Facebook also claims that it was on for the sole purpose of making sure the transcription application was performing properly.
Needless to say, it is difficult to believe anything Facebook has to say on this or any other privacy issue. Even worse, third-parties were used to transcribe the audio, but this information was never disclosed to users.
This is not going to help Facebook in regards to the Libra cryptocurrency venture. Cryptocurrency users love the idea that transactions are relatively private and like to hope and believe that what they do with their transactions is not scrutinized.