Also, they are listening to your microphone.
You might think that Facebook knows about what you’re thinking about buying on Amazon because you’re talking to your spouse out loud about buying that thing. But more likely than not, it’s because you also mentioned it on Facebook Messenger — or Instagram, or WhatsApp. Or, because you already searched for the thing on Amazon, and Amazon sold that data to Facebook. Or because you watched a video about it on YouTube, or read a review article about it (which is what inspired you to be interested in buying it in the first place), and YouTube or that media conglomerate sold your data to Facebook/Amazon/etc.
Likewise, if Amazon knows about your Facebook activity, that’s likely an example of data-sharing in the opposite direction — Facebook selling to Amazon.
Of course, both Amazon and Facebook also partner with tons of other smaller third parties as well, to do ad-tech targeting that those third parties don’t bother to do themselves.
Something people always wonder about is how Tumblr still exists, despite having the worst internal ad-tech ever. Well, they don’t need to make money showing you ads themselves; they can just sell your data to Facebook/Amazon/etc., to improve their ad targeting of you.
Same goes for, say, Wordpress. Just because a blog doesn’t have ads on it, doesn’t mean it isn’t collecting your data and feeding it into one (or multiple) of the big five’s ad-tech backends.
Why do you say that it's a superstition? Have you reverse-engineered the Facebook, Instagram, and Messenger apps?
This indicator has not been reported to appear "for no reason" in these apps.
(Mind you, there's no similar feature for Android, so FAANG might be carefully snooping on only Android users. But it's pretty unlikely.)
Also, people snoop the network traffic from these apps all the time. There's no raw audio in there when there shouldn't be. (Or do you imagine the hypothetical speech-to-text modelling is happening on your phone? That'd be nigh-on Pokemon-Go-like in its battery draw. For an example of a similar battery draw: try opening Shazam or another "audio fingerprinting" app, and just continuously re-activating it whenever it deactivates. You'll lose a lot of battery, pretty quickly.)
Android has about two billion users - there's an extremely strong incentive for large, ad-driven companies (like Facebook) to segment Android off and take advantage of every API possible to gather data with - and they certainly have the resources.
> Or do you imagine the hypothetical speech-to-text modelling is happening on your phone?
Yes. Speech recognition is a relatively easy problem for the purpose of the advertiser, because they don't need anything approaching perfect accuracy - unlike tools like, say, Siri, which need a very high accuracy rate. It's well-known that you can sacrifice accuracy for performance/power in many algorithms, and voice recognition has been the focus of intense study, with many low-power algorithms being developed. For instance, here's a design (it does contain a custom chip, although one tested at a large process node and containing functionality similar to a GPU) that consumes 8mW of power on a >100k word dictionary: https://groups.csail.mit.edu/sls/publications/2018/Price_IEE...
> For an example of a similar battery draw: try opening Shazam or another "audio fingerprinting" app
Those are completely different tasks. Audio fingerprinting is not remotely comparable to low-accuracy voice transcription - otherwise Siri and Google Assistant would drain your battery constantly at the same rate.
It's pretty clear that your claims of "superstition" are unfounded speculation that is inconsistent with the abilities of modern technology.
Essentially - Talk to your friends about the most repulsive YouTube ads you’ve seen, loudly, in the presence of their mobile phones.
I ended up doing this after being forced to endure a particularly stomach churning ad related to ear wax removal.
When I discussed it with two friends, I told them about the ad, and wondered if they would start seeing it. Sure enough a day later my friend told me she’s getting the ear wax ad.
Maybe it has to do with android vs iOS, maybe there is some other way the data was linked, maybe it isn’t facebook listening in but some other app.
But it really does look like the microphones are on, and the networks are listening.
For instance, on iOS, they cannot access your microphone without your express permission (though once they have obtained it once, you'd have to go into the Settings app and explicitly revoke it), and they cannot do so while the app is not open.
Similarly, in Safari on macOS, any website needs to ask permission to access the microphone/webcam, and you can revoke the permission if you want. (This isn't quite as obvious, but if you add the Website Preferences button to your toolbar, it becomes much moreso: I strongly recommend doing this for anyone using Safari.)
I know there's a fairly widespread idea that all smartphones act as always-on microphones and cameras for the shady tech overlords (or the government; whichever the person's particular fear is about), but the whole point of Apple's stance on privacy is preventing that sort of invasive, pervasive, surveillance from taking place from iPhones and other Apple devices.