Or talking to a friend on your mobile phone. Your phone company is not allowed to wire tap the conversation.
If you were voice chatting over Facebook they would probably have to make you aware if they were listening/processing the data for words. By make you aware, it probably means bury it in the ToS on page 784 in a generic and ambiguous way and that is sufficient for court should someone litigate.
Now when you start looking at voice assistants like Alexa, GA, etc. the waters could get muddier.