It's an experience many of us feel like we've had: we'll mention something in a face-to-face conversation or phone call, only to see an advert for it on Facebook.
In some cases, there appears to be no other possibility: Facebook is somehow listening in on our conversations, processing what we are saying, and then using that information to feed us with ads.
There are countless examples: of individuals who saw engagement adverts minutes after proposing, of seeing adverts for cat food just after chatting about cats, or of promotions for concerts and films after a face-to-face discussion.
Tech consultant Damian Le Nouaille has even claimed Instagram, owned by Facebook, is listening to him in multiple languages, showing him adverts in English after he spoke about certain products in French or Spanish. Some report seeing these adverts before any posts about the topic, and without having searched or browsed for it before.
This had led many to believe that the adverts, surfaced just minutes after a conversation, are too much of a coincidence. There can only be one conclusion: Facebook and Instagram are listening to us.
Facebook has repeatedly denied eavesdropping on conversations through the phone's microphone.
Last year, after claims that the company was listening in went viral, it explicitly stated: "Facebook does not use your phone's microphone for ads or news feed stories.
"Some recent articles have suggested that we must be listening to people’s conversations in order to show them relevant ads. This is not true. We show ads based on people’s interests and other profile information – not what you’re talking out loud about."
The company once again denied the claims over the weekend. Rob Goldman, who runs advertising technology at Facebook, insisted: "We don't - and have never - used your microphone for ads." Andrew Bosworth, who used to run its advertising business, also denied it.
Why does Facebook need the microphone?
On both Android and iOS, Facebook and Instagram do use the phone's microphone - like any other app, the user must explicitly opt in to give an app permission.
The main reason it does this is so it can record videos, such as Facebook Live or Instagram Stories. Facebook also has a feature in the US, which it has not introduce elsewhere, that can listen to background music or movies so you can add what you are listening to or watching to a status update.
However, this is only activated when a person has the Facebook app open and deliberately typing a status update. Facebook has denied it is used for listening to conversations and that it does not "tag" your profile with the data, only using it to build up a chart of the most listened songs.
Could Facebook or Instagram be surreptitiously listening to you and not tell you about it? In theory, it is possible. iPhone apps can turn on the microphone at any time without informing you, as a researcher pointed out last week. However, it can only do this when the app is open, and this would also break App Store guidelines. Given that Facebook is the biggest app in the world, it probably receives plenty of scrutiny from Apple and Google.
So why does it show me ads about my conversations?
The most likely explanation is it simply doesn't: it's a coincidence, or your imagination. A normal internet user sees hundreds, if not thousands, of adverts every day, and a lot of those will be on Instagram or Facebook: the average user spends almost an hour on Facebook-owned apps a day.
You probably ignore most of those adverts, particularly the ones that are irrelevant. But every now and then one might come out of the blue that you happen to have had a recent conversation about. Because it seems a little too accurate, and that conversation is front of mind, it's that particular advert you twig, while ignoring the thousands that are not about recent conversations.
There's a name for this: the Baader-Meinhof phenomenon, also known as the frequency illusion, and it's a concept that existed long before Facebook existed. If you've ever noticed that you learn a new word or cultural reference, only to then see it used constantly, it's the same feeling. It's a form of cognitive bias - our tendency to assign more importance to things than they deserve.
There's another thing at work here, though. Adverts are targeted based on many different criteria - your browsing history, your Facebook interests and so on. Even if you don't feel that you've given Facebook enough data to target an advert about cat food without it eavesdropping on you, its algorithms may well be sophisticated enough, based on a number of data points, to suggest that you might be the kind of person thinking about getting a cat.
Facebook has some of the brightest computer engineers in the world at work on developing these algorithms, and it's often said that the social network knows you better than you know yourself.
It might well be creepy that Facebook knows this much about you, but it doesn't mean it is listening.
How to turn Facebook's microphone access off
Still not convinced? You can turn off Facebook and Instagram's access to the camera
In iOS, go to Settings -> Privacy - > Microphone and unselect Facebook.
On Android, go to Settings -> Personal -> Privacy and Safety -> App permissions -> Microphone and unselect Facebook.