I don't understand how they are actually doing this. Unless facebook/google/amazon are actively sharing microphone data with them, how are get they getting the data?
Amazon are especially zealous of making sure no-one else can steal data from them. Not because of privacy concerns, but because they want to use that data first.
Well I would guess its an app. Spend some money and make a pretty, shiny or even actually useful app. Market it. Get people to download and install it (possibly for free), and ask for permission to access the microphone.
If this is the case I'm not sure what the fb/google/az connection would be. Perhaps, they could be buying the resulting data.
It would sort of makes sense if you installed a voice-note taking app and it asked for access to the microphone. However, when you are not using that app - is it still using the microphone? Who knows? I do get somewhat irritated with Android's inability to report which apps are doing what at any particular moment.
These are deals done straight with the phone makers. Full audio is never shared with advertised, just the filtered data, in text, ex: "I'm moving to Canada next week" or "My girlfriend just got pregnant"
Full transcription is really difficult to do at the edge on battery power. It's either bandwith heavy or CPU heavy. Both of which nail the battery. The only really practical way I can see it being done is with keyword tracking, and then recording snippets either side.
But that still takes a reasonable amount of power to do.
Amazon are especially zealous of making sure no-one else can steal data from them. Not because of privacy concerns, but because they want to use that data first.