Is your phone listening in? Your stories
Facebook has always denied using smartphone microphones to gather audio from people's real-life conversations and then use that data to serve advertising.
Last week Facebook's vice-president of ads, Rob Goldman, said on Twitter that the technology giant did not and had never carried out the practice.
"Just not true," he tweeted in response to a call for people's stories of occasions when they believed it had happened.
Given the amount of online advertising that people see every day, there is a strong argument that it is pure coincidence – that perhaps the advert has been served before unobserved and is only now noticeable because of an unrelated discussion.
However, there are those who are convinced that they have experienced it. Here are some of the stories you told us.
We saw wedding ads before we announced our engagement
"My fiancee and I both had wedding ads the day after we got engaged, before we had told anyone," said Nate, from Springfield, in the US.
"We bought the ring in the spur of the moment and never looked anything related up.
"And even two weeks ago my fiancee and I went to a friend's and drank a certain kind of liquor neither of us had ever bought or talked about on the phone, and the next morning it was the first ad Facebook showed her."
My hearing aid kept connecting to my phone
"In 2016, I lost the hearing in my right ear. I was given a 'made for iPhone' compatible hearing aid," said Jon, in the US.
"This meant I could take calls, stream music, et cetera from my new hearing aid.
"Whenever the phone connected to the hearing aid, there would be an audible change, like a little click, because it would be switching from streaming the world around me to audio from the hearing aid.
"This worked in reverse too, with a microphone.
"In both the Facebook Messenger app for iOS and the main Facebook app, I would hear that click, the flipping over of the audio source for my hearing aid, at inexplicable times, including when I had audio from apps disabled."
My jokey career move appeared in an ad
"Just last week I left my employment and was sat with my friend having a chat about which direction I was going to go in," said Lindsey, in Lincoln.
"I said, 'I like coffee, I might just end up at Starbucks so I can drink more coffee.'
"Next time I checked my Facebook on my phone, [I saw] a Starbucks advert as they were holding an open event in London to find new staff."
Bin storage suddenly popped up after a chat
"I recently had an extremely bizarre and unsettling experience with some targeted advertising that felt like more than coincidence, after a voice call on WhatsApp," said Olivia, from Austin.
"I was catching up with a friend who lives in London and she told me a story about how her new landlord purchased an outdoor storage shed for their bins (a product that I think is rather uniquely British).
"We had a good laugh about that, and I expressed how I needed something similar here in Texas.
"The next day as I was scrolling through my Facebook newsfeed I came across an ad from Wayfair advertising a storage shed for outdoor bins and was immediately taken aback.
"Before our conversation happened, this was not a product that I even knew existed."
Two mattress ads in five minutes
"Debated this at a bar with some friends… to test it and settle the debate, we got a random product type from another table: a new mattress," said Justin, in Atlanta.
"As I had not bought, searched for, or even thought about a new bed in several years and I couldn't remember seeing a mattress ad online ever, we started talking about beds and mattresses and guessing keywords, like slipping 'California king' and 'buy a mattress online' into the conversation, while intermittently scrolling facebook.
"Two mattress ads in five minutes. None before that conversation."
Ads appeared in Spanish
"I work in the mobile industry and I've noticed this happening more and more over the last few years," wrote Michael, from Grimsby.
"To put it to the test I started learning Spanish on Duolingo, and within one day I had adverts for normal products in Spanish! It was absolutely unbelievable."
Home security became an ad
"I visited a friend who was setting up security cameras at her house," Melissa, from Australia, wrote.
"I have never used the internet to look at anything remotely linked to home security, yet less than an hour after discussing how to set up the cameras, I had a Facebook ad for home security cameras.
"My phone had been in my pocket the whole time."
Eyesight ads but I have perfect vision
"Once, my friend was over and he discussed that he needed Lasik eye surgery," said Austin, from Tigard, Oregon.
"Immediately after, I went on Facebook and a Lasik advertisement appeared.
"I have perfect eyesight, have never searched Lasik ever before."
Bug killer I'd never seen before
"I saw a product at school called the Bug-a-salt salt shooting gun, used for killing flies," said Peter, "something I'd never seen before, nor have I searched for any related products.
"That night I was telling my wife about the product in a personal conversation.
"The following morning after the conversation with my wife about the product, my Facebook opened with an ad from Amazon with the Bug-a-salt fly killer gun listed as 'things you may be interested in'."
It's happened more than once
"I have experienced several instances of ads that relate to a conversation that I was having or had just finished," said Faris, from Cairo.
"I was talking to my wife about a second child, and on that day there were ads related to the topic.
"We talked about a friend's brother who had passed away, and immediately one of the guys we were talking to found ads for undertakers.
"We were talking about the possibility of a Six Flags theme park being opened in Egypt, and on my Facebook that night were ads for Six Flags."