Experience is the best evidence. Give me a better reasonable explanation then? Why do you think it's a conspiracy theory? There are lawsuits[1] and news articles on this.
Someone really screwed up at facebook? Have you read the news recently? They lie through their teeth to their owm shareholders and customers!
If I had to guess they merely transcribe audio on the device and send keywords for ad targeting and other nefarious uses. They can claim there is no "recording" since audio isn't stored anywhere. Honestly,I think this is fairly harmless if you compare it to some of the other more serious conspiracy theories.
Remember how NSA's surveillance was also a conspiracy theory?
IMO,it's a double edged sword for them,they do this sort of crooked business for law enforcent and intelligence community but they also profit by exploiting this for revenue generation.
[1] http://digg.com/2017/facebook-phone-microphone-ads