This happens because of the tracking technology which Facebook supplies to many commercial websites. On Tech Tent this week, we ask how users will react when they discover just how much data about their online lives is being supplied to the social media giant.

In what it is touting as a groundbreaking exercise in transparency, Facebook has unveiled a tool which will allow users to see what kind of data about them external websites are sending to the platform.

When the Off Facebook Activity setting is rolled out, users will be able to disconnect the flow of data, making it less easy for firms to target them with personalised advertising.

David Baser, director of product management at Facebook, tells Tech Tent the company hopes this will dispel some myths about the way people are tracked: "A lot of people, while they are perhaps aware that some data is maybe following them around the internet, don't have a good understanding as to why or how that's happening. And we believe that that may feed the misperception, the myth, that Facebook, for example, might be listening to your microphone."

Facebook accepts that some people may react by disconnecting all of this data from their profiles, and that could have an effect on its revenues. But David Baser says that research it conducted into the likely behaviour of users of the new setting showed they were likely to pick and choose: "They wanted the good offers and deals and coupons that come from most of the businesses. But some of the businesses they didn't want to have sending data about them."

So how are advertisers going to react if they no longer have access to quite such granular data about individual consumers?

 The trend of targeting niche audiences online may be changing

Mat Morrison, planning director at marketing agency Digital Whiskey, admits advertisers may not relish the prospect: "We've become very used over the last 10 years not to target people based on their presumed interest based on their context - what they're reading - but to target them based on what we know about them."

But he says the mood is changing as the marketing industry reflects on whether it has become too obsessed with targeting. Even if Facebook stops allowing them to reach 23-year-old windsurfers in Wyoming quite so effectively, it will retain its attractions as an advertising platform simply because its audience is so huge.

For the last decade advertisers have been moving some of their spending from TV to online platforms in search of better targeting. But Morrison says that trend is likely to continue: "The story that really worried people wasn't that TV advertising doesn't work - we know it works. It was the fact that people weren't watching TV in the same way anymore. And these big platforms can give us TV-size audiences quite effectively."

It will be some time before we know the impact of this new transparency initiative from Facebook. The Off Facebook Activity setting is only available in Ireland, Spain and South Korea for now and will roll out globally over the coming months.

Also on this week's programme:

• As YouTube follows Facebook and Twitter in deleting accounts suspected of spreading misinformation about the Hong Kong protests, we hear how Chinese social media platforms are reacting. Millions of posts on Sina Weibo and similar sites are accusing the Western tech giants of hypocrisy in shutting down free speech - but is this in turn more Chinese government propaganda?

• And 3D printing, a top technology trend just a few years ago, seems to have faded from public consciousness. But we meet Hexr, a British start-up firm 3D printing cycle helmets. Its founders insist the technology is quietly revolutionising manufacturing.